Manipulation of Ammonium Transporters (AMTs) to Improve NUE in Higher Plants

ABSTRACT

The present invention provides polynucleotides and related polypeptides of the protein AMT. The invention provides genomic sequence for the AMT gene. AMT is responsible for controlling nitrogen utilization efficiency in plants.

CROSS REFERENCE

This utility application is a continuation of U.S. patent applicationSer. No. 13/551,169 filed Jul. 17, 2012 which claims benefit of U.S.patent application Ser. No. 13/043,109 filed Mar. 8, 2011, now U.S. Pat.No. 8,252,979 issued on Aug. 28, 2012, which claims the benefit of U.S.patent application Ser. No. 12/045,098 filed Mar. 10, 2008 which claimsthe benefit of U.S. Provisional Application No. 60/893,901, filed Mar.9, 2007, all of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates generally to the field of molecular biology.

BACKGROUND OF THE INVENTION

Nitrogen (N) is the most abundant inorganic nutrient taken up from thesoil by plants for growth and development. Maize roots absorb most ofthe N from the soil in the form of nitrate, the majority of which istransported to the leaf for reduction and assimilation. Nitrate isreduced to nitrite by nitrate reductase (NR) in the cytosol and thennitrite is transported into chloroplast where it is reduced by nitritereductase (NiR) to ammonium. Ammonium is assimilated into glutamine bythe glutamine synthase-glutamate synthase system (Crawford and Glass,(1998) Trends in Plant Science 3:389-395.). Also, it has long been knownthat significant amounts of N are lost from the plant aerial parts byvolatilization (Glyan'ko, et al., (1980) Agrokhimiya 8:19-26; Hooker, etal., (1980) Agronomy Journal 72(5):789-792; Silva, et al., (1981) CropScience 21(6): 913-916; Stutte, et al., (1981) Crop Science21(4):596-600; Foster, et al., (1986) Annals of Botany 57(3):305-307;Parton, et al., (1988) Agronomy Journal 80(3):419-425; Kamiji, et al.,(1989) Japanese Journal of Crop Science 58(1):140-142; Morgan, et al.,(1989) Crop Science 29(3):726-731; O'Deen, (1989) Agronomy Journal81(6):980-985; Guindo, et al., (1994) Arkansas Farm Research43(1):12-13; Heckathorn, et al., (1995) Oecologia 101(3):361-365;Cabezas, et al., (1997) Revista Brasileira de Ciencia do Solo21(3):481-487). Experimental evidence supports the loss of N throughammonium and not through N oxides (Hooker, et al., 1980). Treatment withchemicals that inhibit glutamine or glutamate synthase activities led toincreased loss of ammonium through volatilization (Foster, et al.,1986). Loss of N is not only limited to C-3 species as C-4 plants havealso been reported to lose N through volatilization (Heckathorn, et al.,1995).

Manipulation of AMTs can be utilized to improve NUE by causing increaseddry matter, thereby contributing to an increase in plant yield. Two ofthe ways to improved dry matter accumulation are: 1) reduce N lossthrough volatilization and 2) reduce N content of the plant so that moredry matter can be accumulated in the form of low-energy constituents,e.g., starch or cellulose.

For ammonium to be lost from the leaf, it must first pass through afacilitated channel since it is highly hydrophilic. Ammoniumtransporters (AMTs) were originally discovered as ammonium transportersbut some recent studies have shown that at least in some cases AMTs canact as gas channels (Soupene, et al., (2002) Proc Natl Acad Sci USA99:3926-3931; Kustu and Inwood, (2006) Transfus Clin Biol 13:103-110).An amtB knock-out mutant of Salmonella grows better on poor N source,apparently because it can sequester more N by keeping it from leakingback out (Soupene, et al., 2002). This application details an inventionwhich is used to manipulate AMTs in higher plants to improve NUE. Theinventors identified chloroplast-specific and/or leaf-preferred AMT(s)and knocked them out/down to minimize the loss of ammonium, whichresulting in better N assimilation/NUE. In addition, work was notlimited only to the chloroplast-localized AMTs but will alsodown-regulation of the AMTs that are localized to otherorganelles/membranes.

SUMMARY OF THE INVENTION

The present invention provides polynucleotides, related polypeptides andall conservatively modified variants of the present AMT sequences. Theinvention provides sequences for the AMT genes. Six Arabidopsis, 7maize, 17 rice and 11 soybean AMT genes were identified. Table 1 liststhese genes and their seq id numbers.

TABLE 1 SEQUENCE ID NUMBER IDENTITY SEQ ID NOS: 1 AtAMT 1 polynucleotideSEQ ID NOS: 2 AtAMT 1 polypeptide SEQ ID NO: 3 AtAMT 1; 2 polynucleotideSEQ ID NO: 4 AtAMT 1; 2 polypeptide SEQ ID NO: 5 AtAMT 1; 3polynucleotide SEQ ID NO: 6 AtAMT 1; 3 polypeptide SEQ ID NO: 7 AtAMT 2polynucleotide SEQ ID NO: 8 AtAMT 2 polypeptide SEQ ID NO: 9 AtAMT 3polynucleotide SEQ ID NO: 10 AtAMT 3 polypeptide SEQ ID NO: 11 AtAMT 4polynucleotide SEQ ID NO: 12 AtAMT 4 polypeptide SEQ ID NO: 13 ZmAMT 1polynucleotide SEQ ID NO: 14 ZmAMT 1 polypeptide SEQ ID NO: 15 ZmAMT 2polynucleotide SEQ ID NO: 16 ZmAMT 2 polypeptide SEQ ID NO: 17 ZmAMT 3polynucleotide SEQ ID NO: 18 ZmAMT 3 polypeptide SEQ ID NO: 19 ZmAMT 4polynucleotide SEQ ID NO: 20 ZmAMT 4 polypeptide SEQ ID NO: 21 ZmAMT 5polynucleotide SEQ ID NO: 22 ZmAMT 5 polypeptide SEQ ID NO: 23 ZmAMT 6polynucleotide SEQ ID NO: 24 ZmAMT 6 polypeptide SEQ ID NO: 25 ZmAMT 7polynucleotide SEQ ID NO: 26 ZmAMT 7 polypeptide SEQ ID NO: 27 OsAMT 1polynucleotide SEQ ID NO: 28 OsAMT 1 polypeptide SEQ ID NO: 29 OsAMT 2polynucleotide SEQ ID NO: 30 OsAMT 2 polypeptide SEQ ID NO: 31 OsAMT 3polynucleotide SEQ ID NO: 32 OsAMT 3 polypeptide SEQ ID NO: 33 OsAMT 4polynucleotide SEQ ID NO: 34 OsAMT 4 polypeptide SEQ ID NO: 35 OsAMT 5polynucleotide SEQ ID NO: 36 OsAMT 5 polypeptide SEQ ID NO: 37 OsAMT 6polynucleotide SEQ ID NO: 38 OsAMT 6 polypeptide SEQ ID NO: 39 OsAMT 7polynucleotide SEQ ID NO: 40 OsAMT 7 polypeptide SEQ ID NO: 41 OsAMT 8polynucleotide SEQ ID NO: 42 OsAMT 8 polypeptide SEQ ID NO: 43 OsAMT 9polynucleotide SEQ ID NO: 44 OsAMT 9 polypeptide SEQ ID NO: 45 OsAMT 10polynucleotide SEQ ID NO: 46 OsAMT 10 polypeptide SEQ ID NO: 47 OsAMT 11polynucleotide SEQ ID NO: 48 OsAMT 11 polypeptide SEQ ID NO: 49 OsAMT 12polynucleotide SEQ ID NO: 50 OsAMT 12 polypeptide SEQ ID NO: 51 OsAMT 13polynucleotide SEQ ID NO: 52 OsAMT 13 polypeptide SEQ ID NO: 53 OsAMT 14polynucleotide SEQ ID NO: 54 OsAMT 14 polypeptide SEQ ID NO: 55 OsAMT 15polynucleotide SEQ ID NO: 56 OsAMT 15 polypeptide SEQ ID NO: 57 OsAMT 16polynucleotide SEQ ID NO: 58 OsAMT 16 polypeptide SEQ ID NO: 59 OsAMT 17polynucleotide SEQ ID NO: 60 OsAMT 17 polynucleotide SEQ ID NO: 61 GmAMT1 polynucleotide SEQ ID NO: 62 GmAMT 1 polypeptide SEQ ID NO: 63 GmAMT 2polynucleotide SEQ ID NO: 64 GmAMT 2 polypeptide SEQ ID NO: 65 GmAMT 3polynucleotide SEQ ID NO: 66 GmAMT 3 polypeptide SEQ ID NO: 67 GmAMT 4polynucleotide SEQ ID NO: 68 GmAMT 4 polypeptide SEQ ID NO: 69 GmAMT 5polynucleotide SEQ ID NO: 70 GmAMT 5 polypeptide SEQ ID NO: 71 GmAMT 6polynucleotide SEQ ID NO: 72 GmAMT 6 polypeptide SEQ ID NO: 73 GmAMT 7polynucleotide SEQ ID NO: 74 GmAMT 7 polypeptide SEQ ID NO: 75 GmAMT 8polynucleotide SEQ ID NO: 76 GmAMT 8 polypeptide SEQ ID NO: 77 GmAMT 9polynucleotide SEQ ID NO: 78 GmAMT 9 polypeptide SEQ ID NO: 79 GmAMT 10polynucleotide SEQ ID NO: 80 GmAMT 10 polypeptide SEQ ID NO: 81 GmAMT 11polynucleotide SEQ ID NO: 82 GmAMT 11 polypeptide

Therefore, in one aspect, the present invention relates to an isolatednucleic acid comprising an isolated polynucleotide sequence encoding anAMT protein. One embodiment of the invention is an isolatedpolynucleotide comprising a nucleotide sequence selected from the groupconsisting of: (a) the nucleotide sequence comprising SEQ ID NO: 1, 3,5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41,43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77,79 or 81; (b) the nucleotide sequence encoding an amino acid sequencecomprising SEQ ID NO: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26,28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62,64, 66, 68, 70, 72, 74, 76, 78, 80 or 82 and (c) the nucleotide sequencecomprising at least 70% sequence identity to SEQ ID NO: 1, 3, 5, 7, 9,11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45,47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79 or81, wherein said polynucleotide encodes a polypeptide having AMTtransporter activity.

Compositions of the invention include an isolated polypeptide comprisingan amino acid sequence selected from the group consisting of: (a) theamino acid sequence comprising SEQ ID NO: 2, 4, 6, 8, 10, 12, 14, 16,18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52,54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80 or 82 and (b) theamino acid sequence comprising at least 70% sequence identity to SEQ IDNO: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36,38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72,74, 76, 78, 80 or 82, wherein said polypeptide has AMT transporteractivity.

In another aspect, the present invention relates to a recombinantexpression cassette comprising a nucleic acid as described.Additionally, the present invention relates to a vector containing therecombinant expression cassette. Further, the vector containing therecombinant expression cassette can facilitate the transcription andtranslation of the nucleic acid in a host cell. The present inventionalso relates to the host cells able to express the polynucleotide of thepresent invention. A number of host cells could be used, such as but notlimited to, microbial, mammalian, plant or insect.

In yet another embodiment, the present invention is directed to atransgenic plant or plant cells, containing the nucleic acids of thepresent invention. Preferred plants containing the polynucleotides ofthe present invention include but are not limited to maize, soybean,sunflower, sorghum, canola, wheat, alfalfa, cotton, rice, barley,tomato, switchgrass, myscanthus, triticale and millet. In anotherembodiment, the transgenic plant is a maize plant or plant cells.Another embodiment is the transgenic seeds from the transgenic plant.Another embodiment of the invention includes plants comprising an amtpolypeptide of the invention operably linked to a promoter that drivesexpression in the plant. The plants of the invention can have alteredAMT as compared to a control plant. In some plants, the AMT is alteredin a vegetative tissue, a reproductive tissue, or a vegetative tissueand a reproductive tissue. Plants of the invention can have at least oneof the following phenotypes including but not limited to: increased leafsize, increased ear size, increased seed size, increased endosperm size,alterations in the relative size of embryos and endosperms leading tochanges in the relative levels of protein, oil and/or starch in theseeds, absence of tassels, absence of functional pollen bearing tasselsor increased plant size.

Another embodiment of the invention would be plants that have beengenetically modified at a genomic locus, wherein the genomic locusencodes an amt polypeptide of the invention.

Methods for increasing the activity of an amt polypeptide in a plant areprovided. The method can comprise introducing into the plant an amtpolynucleotide of the invention. Providing the polypeptide can decreasethe number of cells in plant tissue, modulating the tissue growth andsize.

Methods for reducing or eliminating the level of an amt polypeptide inthe plant are provided. The level or activity of the polypeptide couldalso be reduced or eliminated in specific tissues, causing increased AMTin said tissues. Reducing the level and/or activity of the AMTpolypeptide increases the number of cells produced in the associatedtissue.

Compositions further include plants and seed having a DNA constructcomprising a nucleotide sequence of interest operably linked to apromoter of the current invention. In specific embodiments, the DNAconstruct is stably integrated into the genome of the plant. The methodcomprises introducing into a plant a nucleotide sequence of interestoperably linked to a promoter of the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: (as FIG. 1) Phylogentic tree of AMTs from Arabidopsis, rice,soybean and maize

Phylogenetic analyses of all the AMTs from Arabidopsis, rice, maize andsoybean are shown in FIG. 1. The length of the line at the base of thefigure represents an equivalent of 10 amino acid differences and couldbe used to approximate the amino acid differences between differentammonium transporter proteins from the individual branch lengths.

FIG. 2: (as FIG. 2) Expression analysis of ZmAMTs

In order to identify leaf specific/preferred/expressed AMT(s) in maize,Lynx MPSS expression analyses in ˜300 libraries reveal that ZmAMT1 (SEQID NO: 14), 2, 7 are expressed both in roots and leaves whereas ZmAMT4(SEQ ID NO: 20) is a root preferred AMT. ZmAMT6 (SEQ ID NO: 24)expresses at very low level in comparison to other Zm-AMTs. In case ofZmAMT5 there was no specific Lynx tag available.

FIG. 3: (as Fig. A, B and C) Characterization of atamt1;2 T-DNAknock-out mutant

In cTP prediction analyses, AtAMT1;2 (SEQ ID NO: 4) posses a putativecTP. For functional analyses of AtAMT1;2 (SEQ ID NO: 4) and to determineit's role in N-assimilation, analyses identified a T-DNA mutant line(SM_(—)3.15680) from the Arabidopsis T-DNA mutant data base. In thismutant line T-DNA was inserted in c-terminal of AtAMT1;2 (SEQ ID NO: 4)gene (FIG. 3A). Genomic PCRs using AtAMT1;2 (SEQ ID NO: 4) gene andT-DNA specific primers show that T-DNA is indeed inserted in theAtAMT1;2 (SEQ ID NO: 4) (FIG. 3B). AtAMT1;2 (SEQ ID NO: 4) gene specificprimers flanking the T-DNA insert couldn't amplify any DNA region inmutant plants where as an expected PCR product was detected in wild typeplant (FIG. 3B, upper panel). Similarly, genomic PCR with AtAMT1;2 (SEQID NO: 4) specific forward primer and T-DNA specific reverse primersamplify an expected product in mutant lines and nothing in wild typeplants as expected (FIG. 3B, lower panel). Saturated RT-PCRs (35 cycles)analyses couldn't detect a full length atamt1;2 mRNA in mutant (FIG. 3C,upper panel) suggesting that AtAMT1;2 (SEQ ID NO: 4) is completelyknocked out in this T-DNA mutant. Actin control RT-PCR worked fine inboth mutant and wild type plants (FIG. 3C, lower panel).

FIG. 4: (as FIGS. 4A and 4B) Knock-out of multiple AMTs in Arabidopsisby single RNAi vector

Six AMT genes are present in Arabidopsis genome. Hence, it is verylikely that due to functional redundancy one might need to manipulatethe expression of multiple AMTs simultaneously. Analyses of the DNAsequence of all these AMTs was performed which identified the highhomology regions among them. There is a stretch of ˜200 bp amongAtAMT1;2 (SEQ ID NO: 4), AtAMT1 (SEQ ID NO: 2), AMT1;3 (SEQ ID NO: 6),At3g24290 (SEQ ID NO: 10) and At4g28700 (SEQ ID NO: 12) where as AMT2(SEQ ID NO: 8) shown in FIG. 4B stood independent. Amplification ofthese regions was accomplished (bold and underlined in FIG. 4A) by PCRfrom AtAMT1;2 (SEQ ID NO: 4) and AtAMT2 (SEQ ID NO: 8) and a multi-wayligation was performed to make an inverted repeat using ADH-intron as aspacer. The RNAi cassette of these hybrid inverted repeats is driven byconstitutive or root specific or leaf specific promoter.

FIG. 5: (as FIGS. 5A and 5B) Knock-out/down of multiple AMTs in Maize bysingle RNAi vector

Detailed analyses of all 7 maize AMTs were performed to identify the DNAregions showing high homology among different ZmAMTs. This analysisreveals that ZmAMT1 (SEQ ID NO: 14) and ZmAMT5 (SEQ ID NO: 22), ZmAMT3(SEQ ID NO: 18) and ZmAMT4 (SEQ ID NO: 20) and ZmAMT2 (SEQ ID NO: 16),ZmAMT6 (SEQ ID NO: 24) and ZmAMT7 (SEQ ID NO: 26) form three separategroups and there is a very high homology in stretches of DNA sequenceswith in each group. Three DNA fragments (bold and underlined in FIGS. 5Aand B) from ZmAMT 1, 4 and 7 (SEQ ID NOS: 14, 20 and 26) representingeach of the different groups were amplified by PCR. Multi-way ligationswere performed to make inverted repeats with hybrid of these 3 fragmentsand ADH intron as a spacer to facilitate the formation of stem-loopstructure. This RNAi cassette of ‘ZmAMT1 (SEQ ID NO: 14):ZmAMT4 (SEQ IDNO: 20):ZmAMT7 (SEQ ID NO: 26)’ inverted repeats was driven by aconstitutive (Zm-UBI promoter) or leaf-specific promoter. MOPAT drivenby ZmUBI promoter was used as herbicide resistance marker for selected.In addition to that RFP driven by a pericarp specific promoter LTP2 wasalso used to sort out the transgenic seeds (red) from there segregatingnon-transgenic seeds.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Unless mentioned otherwise, thetechniques employed or contemplated herein are standard methodologieswell known to one of ordinary skill in the art. The materials, methodsand examples are illustrative only and not limiting. The following ispresented by way of illustration and is not intended to limit the scopeof the invention.

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

The practice of the present invention will employ, unless otherwiseindicated, conventional techniques of botany, microbiology, tissueculture, molecular biology, chemistry, biochemistry and recombinant DNAtechnology, which are within the skill of the art. Such techniques areexplained fully in the literature. See, e.g., Langenheim and Thimann,BOTANY: PLANT BIOLOGY AND ITS RELATION TO HUMAN AFFAIRS, John Wiley(1982); CELL CULTURE AND SOMATIC CELL GENETICS OF PLANTS, vol. 1, Vasil,ed. (1984); Stanier, et al., THE MICROBIAL WORLD, 5^(th) ed.,Prentice-Hall (1986); Dhringra and Sinclair, BASIC PLANT PATHOLOGYMETHODS, CRC Press (1985); Maniatis, et al., MOLECULAR CLONING: ALABORATORY MANUAL (1982); DNA CLONING, vols. I and II, Glover, ed.(1985); OLIGONUCLEOTIDE SYNTHESIS, Gait, ed. (1984); NUCLEIC ACIDHYBRIDIZATION, Hames and Higgins, eds. (1984); and the series METHODS INENZYMOLOGY, Colowick and Kaplan, eds, Academic Press, Inc., San Diego,Calif.

Units, prefixes and symbols may be denoted in their SI accepted form.Unless otherwise indicated, nucleic acids are written left to right in5′ to 3′ orientation; amino acid sequences are written left to right inamino to carboxy orientation, respectively. Numeric ranges are inclusiveof the numbers defining the range. Amino acids may be referred to hereinby either their commonly known three letter symbols or by the one-lettersymbols recommended by the IUPAC-IUB Biochemical NomenclatureCommission. Nucleotides, likewise, may be referred to by their commonlyaccepted single-letter codes. The terms defined below are more fullydefined by reference to the specification as a whole.

In describing the present invention, the following terms will beemployed, and are intended to be defined as indicated below.

By “microbe” is meant any microorganism (including both eukaryotic andprokaryotic microorganisms), such as fungi, yeast, bacteria,actinomycetes, algae and protozoa, as well as other unicellularstructures.

By “amplified” is meant the construction of multiple copies of a nucleicacid sequence or multiple copies complementary to the nucleic acidsequence using at least one of the nucleic acid sequences as a template.Amplification systems include the polymerase chain reaction (PCR)system, ligase chain reaction (LCR) system, nucleic acid sequence basedamplification (NASBA, Cangene, Mississauga, Ontario), Q-Beta Replicasesystems, transcription-based amplification system (TAS), and stranddisplacement amplification (SDA). See, e.g., DIAGNOSTIC MOLECULARMICROBIOLOGY: PRINCIPLES AND APPLICATIONS, Persing, et al., eds.,American Society for Microbiology, Washington, D.C. (1993). The productof amplification is termed an amplicon.

The term “conservatively modified variants” applies to both amino acidand nucleic acid sequences. With respect to particular nucleic acidsequences, conservatively modified variants refer to those nucleic acidsthat encode identical or conservatively modified variants of the aminoacid sequences. Because of the degeneracy of the genetic code, a largenumber of functionally identical nucleic acids encode any given protein.For instance, the codons GCA, GCC, GCG and GCU all encode the amino acidalanine. Thus, at every position where an alanine is specified by acodon, the codon can be altered to any of the corresponding codonsdescribed without altering the encoded polypeptide. Such nucleic acidvariations are “silent variations” and represent one species ofconservatively modified variation. Every nucleic acid sequence hereinthat encodes a polypeptide also describes every possible silentvariation of the nucleic acid. One of ordinary skill will recognize thateach codon in a nucleic acid (except AUG, which is ordinarily the onlycodon for methionine; one exception is Micrococcus rubens, for which GTGis the methionine codon (Ishizuka, et al., (1993) J. Gen. Microbiol.139:425-32) can be modified to yield a functionally identical molecule.Accordingly, each silent variation of a nucleic acid, which encodes apolypeptide of the present invention, is implicit in each describedpolypeptide sequence and incorporated herein by reference.

As to amino acid sequences, one of skill will recognize that individualsubstitutions, deletions or additions to a nucleic acid, peptide,polypeptide or protein sequence which alters, adds or deletes a singleamino acid or a small percentage of amino acids in the encoded sequenceis a “conservatively modified variant” when the alteration results inthe substitution of an amino acid with a chemically similar amino acid.Thus, any number of amino acid residues selected from the group ofintegers consisting of from 1 to 15 can be so altered. Thus, forexample, 1, 2, 3, 4, 5, 7 or 10 alterations can be made. Conservativelymodified variants typically provide similar biological activity as theunmodified polypeptide sequence from which they are derived. Forexample, substrate specificity, enzyme activity, or ligand/receptorbinding is generally at least 30%, 40%, 50%, 60%, 70%, 80% or 90%,preferably 60-90% of the native protein for it's native substrate.Conservative substitution tables providing functionally similar aminoacids are well known in the art.

The following six groups each contain amino acids that are conservativesubstitutions for one another:

1) Alanine (A), Serine (S), Threonine (T);

2) Aspartic acid (D), Glutamic acid (E);

3) Asparagine (N), Glutamine (Q);

4) Arginine (R), Lysine (K);

5) Isoleucine (I), Leucine (L), Methionine (M), Valine (V); and

6) Phenylalanine (F), Tyrosine (Y), Tryptophan (W).

See also, Creighton, PROTEINS, W.H. Freeman and Co. (1984).

As used herein, “consisting essentially of” means the inclusion ofadditional sequences to an object polynucleotide where the additionalsequences do not selectively hybridize, under stringent hybridizationconditions, to the same cDNA as the polynucleotide and where thehybridization conditions include a wash step in 0.1×SSC and 0.1% sodiumdodecyl sulfate at 65° C.

By “encoding” or “encoded,” with respect to a specified nucleic acid, ismeant comprising the information for translation into the specifiedprotein. A nucleic acid encoding a protein may comprise non-translatedsequences (e.g., introns) within translated regions of the nucleic acid,or may lack such intervening non-translated sequences (e.g., as incDNA). The information by which a protein is encoded is specified by theuse of codons. Typically, the amino acid sequence is encoded by thenucleic acid using the “universal” genetic code. However, variants ofthe universal code, such as is present in some plant, animal, and fungalmitochondria, the bacterium Mycoplasma capricolumn (Yamao, et al.,(1985) Proc. Natl. Acad. Sci. USA 82:2306-9) or the ciliateMacronucleus, may be used when the nucleic acid is expressed using theseorganisms.

When the nucleic acid is prepared or altered synthetically, advantagecan be taken of known codon preferences of the intended host where thenucleic acid is to be expressed. For example, although nucleic acidsequences of the present invention may be expressed in bothmonocotyledonous and dicotyledonous plant species, sequences can bemodified to account for the specific codon preferences and GC contentpreferences of monocotyledonous plants or dicotyledonous plants as thesepreferences have been shown to differ (Murray, et al., (1989) NucleicAcids Res. 17:477-98 and herein incorporated by reference). Thus, themaize preferred codon for a particular amino acid might be derived fromknown gene sequences from maize. Maize codon usage for 28 genes frommaize plants is listed in Table 4 of Murray, et al., supra.

As used herein, “heterologous” in reference to a nucleic acid is anucleic acid that originates from a foreign species, or, if from thesame species, is substantially modified from its native form incomposition and/or genomic locus by deliberate human intervention. Forexample, a promoter operably linked to a heterologous structural gene isfrom a species different from that from which the structural gene wasderived or, if from the same species, one or both are substantiallymodified from their original form. A heterologous protein may originatefrom a foreign species or, if from the same species, is substantiallymodified from its original form by deliberate human intervention.

By “host cell” is meant a cell, which comprises a heterologous nucleicacid sequence of the invention, which contains a vector and supports thereplication and/or expression of the expression vector. Host cells maybe prokaryotic cells such as E. coli, or eukaryotic cells such as yeast,insect, plant, amphibian or mammalian cells. Preferably, host cells aremonocotyledonous or dicotyledonous plant cells, including but notlimited to maize, sorghum, sunflower, soybean, wheat, alfalfa, rice,cotton, canola, barley, millet, switchgrass, myscanthus, triticale andtomato. A particularly preferred monocotyledonous host cell is a maizehost cell.

The term “hybridization complex” includes reference to a duplex nucleicacid structure formed by two single-stranded nucleic acid sequencesselectively hybridized with each other.

The term “introduced” in the context of inserting a nucleic acid into acell, means “transfection” or “transformation” or “transduction” andincludes reference to the incorporation of a nucleic acid into aeukaryotic or prokaryotic cell where the nucleic acid may beincorporated into the genome of the cell (e.g., chromosome, plasmid,plastid or mitochondrial DNA), converted into an autonomous replicon ortransiently expressed (e.g., transfected mRNA).

The terms “isolated” refers to material, such as a nucleic acid or aprotein, which is substantially or essentially free from componentswhich normally accompany or interact with it as found in its naturallyoccurring environment. The isolated material optionally comprisesmaterial not found with the material in its natural environment. Nucleicacids, which are “isolated”, as defined herein, are also referred to as“heterologous” nucleic acids. Unless otherwise stated, the term “AMTnucleic acid” means a nucleic acid comprising a polynucleotide (“AMTpolynucleotide”) encoding a full length or partial length AMTpolypeptide.

As used herein, “nucleic acid” includes reference to adeoxyribonucleotide or ribonucleotide polymer in either single- ordouble-stranded form, and unless otherwise limited, encompasses knownanalogues having the essential nature of natural nucleotides in thatthey hybridize to single-stranded nucleic acids in a manner similar tonaturally occurring nucleotides (e.g., peptide nucleic acids).

By “nucleic acid library” is meant a collection of isolated DNA or RNAmolecules, which comprise and substantially represent the entiretranscribed fraction of a genome of a specified organism. Constructionof exemplary nucleic acid libraries, such as genomic and cDNA libraries,is taught in standard molecular biology references such as Berger andKimmel, GUIDE TO MOLECULAR CLONING TECHNIQUES, from the series METHODSIN ENZYMOLOGY, vol. 152, Academic Press, Inc., San Diego, Calif. (1987);Sambrook, et al., MOLECULAR CLONING: A LABORATORY MANUAL, 2^(nd) ed.,vols. 1-3 (1989); and CURRENT PROTOCOLS IN MOLECULAR BIOLOGY, Ausubel,et al., eds, Current Protocols, a joint venture between GreenePublishing Associates, Inc. and John Wiley & Sons, Inc. (1994Supplement).

As used herein “operably linked” includes reference to a functionallinkage between a first sequence, such as a promoter, and a secondsequence, wherein the promoter sequence initiates and mediatestranscription of the DNA corresponding to the second sequence.Generally, operably linked means that the nucleic acid sequences beinglinked are contiguous and, where necessary to join two protein codingregions, contiguous and in the same reading frame.

As used herein, the term “plant” includes reference to whole plants,plant organs (e.g., leaves, stems, roots, etc.), seeds and plant cellsand progeny of same. Plant cell, as used herein includes, withoutlimitation, seeds, suspension cultures, embryos, meristematic regions,callus tissue, leaves, roots, shoots, gametophytes, sporophytes, pollenand microspores. The class of plants, which can be used in the methodsof the invention, is generally as broad as the class of higher plantsamenable to transformation techniques, including both monocotyledonousand dicotyledonous plants including species from the genera: Cucurbita,Rosa, Vitis, Juglans, Fragaria, Lotus, Medicago, Onobrychis, Trifolium,Trigonella, Vigna, Citrus, Linum, Geranium, Manihot, Daucus,Arabidopsis, Brassica, Raphanus, Sinapis, Atropa, Capsicum, Datura,Hyoscyamus, Lycopersicon, Nicotiana, Solanum, Petunia, Digitalis,Majorana, Ciahorium, Helianthus, Lactuca, Bromus, Asparagus,Antirrhinum, Heterocallis, Nemesis, Pelargonium, Panieum, Pennisetum,Ranunculus, Senecio, Salpiglossis, Cucumis, Browaalia, Glycine, Pisum,Phaseolus, Lolium, Oryza, Avena, Hordeum, Secale, Allium and Triticum. Aparticularly preferred plant is Zea mays.

As used herein, “yield” may include reference to bushels per acre of agrain crop at harvest, as adjusted for grain moisture (15% typically formaize, for example). Grain moisture is measured in the grain at harvest.The adjusted test weight of grain is determined to be the weight inpounds per bushel, adjusted for grain moisture level at harvest.

As used herein, “polynucleotide” includes reference to adeoxyribopolynucleotide, ribopolynucleotide or analogs thereof that havethe essential nature of a natural ribonucleotide in that they hybridize,under stringent hybridization conditions, to substantially the samenucleotide sequence as naturally occurring nucleotides and/or allowtranslation into the same amino acid(s) as the naturally occurringnucleotide(s). A polynucleotide can be full-length or a subsequence of anative or heterologous structural or regulatory gene. Unless otherwiseindicated, the term includes reference to the specified sequence as wellas the complementary sequence thereof. Thus, DNAs or RNAs with backbonesmodified for stability or for other reasons are “polynucleotides” asthat term is intended herein. Moreover, DNAs or RNAs comprising unusualbases, such as inosine, or modified bases, such as tritylated bases, toname just two examples, are polynucleotides as the term is used herein.It will be appreciated that a great variety of modifications have beenmade to DNA and RNA that serve many useful purposes known to those ofskill in the art. The term polynucleotide as it is employed hereinembraces such chemically, enzymatically or metabolically modified formsof polynucleotides, as well as the chemical forms of DNA and RNAcharacteristic of viruses and cells, including inter alia, simple andcomplex cells.

The terms “polypeptide,” “peptide” and “protein” are usedinterchangeably herein to refer to a polymer of amino acid residues. Theterms apply to amino acid polymers in which one or more amino acidresidue is an artificial chemical analogue of a corresponding naturallyoccurring amino acid, as well as to naturally occurring amino acidpolymers.

As used herein “promoter” includes reference to a region of DNA upstreamfrom the start of transcription and involved in recognition and bindingof RNA polymerase and other proteins to initiate transcription. A “plantpromoter” is a promoter capable of initiating transcription in plantcells. Exemplary plant promoters include, but are not limited to, thosethat are obtained from plants, plant viruses, and bacteria whichcomprise genes expressed in plant cells such Agrobacterium or Rhizobium.Examples are promoters that preferentially initiate transcription incertain tissues, such as leaves, roots, seeds, fibres, xylem vessels,tracheids or sclerenchyma. Such promoters are referred to as “tissuepreferred.” A “cell type” specific promoter primarily drives expressionin certain cell types in one or more organs, for example, vascular cellsin roots or leaves. An “inducible” or “regulatable” promoter is apromoter, which is under environmental control. Examples ofenvironmental conditions that may effect transcription by induciblepromoters include anaerobic conditions or the presence of light. Anothertype of promoter is a developmentally regulated promoter, for example, apromoter that drives expression during pollen development. Tissuepreferred, cell type specific, developmentally regulated and induciblepromoters constitute the class of “non-constitutive” promoters. A“constitutive” promoter is a promoter, which is active under mostenvironmental conditions.

The term “AMT polypeptide” refers to one or more amino acid sequences.The term is also inclusive of fragments, variants, homologs, alleles orprecursors (e.g., preproproteins or proproteins) thereof. A “AMTprotein” comprises an amt polypeptide. Unless otherwise stated, the term“AMT nucleic acid” means a nucleic acid comprising a polynucleotide(“AMT polynucleotide”) encoding an amt polypeptide.

As used herein “recombinant” includes reference to a cell or vector,that has been modified by the introduction of a heterologous nucleicacid or that the cell is derived from a cell so modified. Thus, forexample, recombinant cells express genes that are not found in identicalform within the native (non-recombinant) form of the cell or expressnative genes that are otherwise abnormally expressed, under expressed ornot expressed at all as a result of deliberate human intervention; ormay have reduced or eliminated expression of a native gene. The term“recombinant” as used herein does not encompass the alteration of thecell or vector by naturally occurring events (e.g., spontaneousmutation, natural transformation/transduction/transposition) such asthose occurring without deliberate human intervention.

As used herein, a “recombinant expression cassette” is a nucleic acidconstruct, generated recombinantly or synthetically, with a series ofspecified nucleic acid elements, which permit transcription of aparticular nucleic acid in a target cell. The recombinant expressioncassette can be incorporated into a plasmid, chromosome, mitochondrialDNA, plastid DNA, virus or nucleic acid fragment. Typically, therecombinant expression cassette portion of an expression vectorincludes, among other sequences, a nucleic acid to be transcribed and apromoter.

The term “residue” or “amino acid residue” or “amino acid” are usedinterchangeably herein to refer to an amino acid that is incorporatedinto a protein, polypeptide or peptide (collectively “protein”). Theamino acid may be a naturally occurring amino acid and, unless otherwiselimited, may encompass known analogs of natural amino acids that canfunction in a similar manner as naturally occurring amino acids.

The term “selectively hybridizes” includes reference to hybridization,under stringent hybridization conditions, of a nucleic acid sequence toa specified nucleic acid target sequence to a detectably greater degree(e.g., at least 2-fold over background) than its hybridization tonon-target nucleic acid sequences and to the substantial exclusion ofnon-target nucleic acids. Selectively hybridizing sequences typicallyhave about at least 40% sequence identity, preferably 60-90% sequenceidentity and most preferably 100% sequence identity (i.e.,complementary) with each other.

The terms “stringent conditions” or “stringent hybridization conditions”include reference to conditions under which a probe will hybridize toits target sequence, to a detectably greater degree than other sequences(e.g., at least 2-fold over background). Stringent conditions aresequence-dependent and will be different in different circumstances. Bycontrolling the stringency of the hybridization and/or washingconditions, target sequences can be identified which can be up to 100%complementary to the probe (homologous probing). Alternatively,stringency conditions can be adjusted to allow some mismatching insequences so that lower degrees of similarity are detected (heterologousprobing). Optimally, the probe is approximately 500 nucleotides inlength, but can vary greatly in length from less than 500 nucleotides toequal to the entire length of the target sequence.

Typically, stringent conditions will be those in which the saltconcentration is less than about 1.5 M Na ion, typically about 0.01 to1.0 M Na ion concentration (or other salts) at pH 7.0 to 8.3 and thetemperature is at least about 30° C. for short probes (e.g., 10 to 50nucleotides) and at least about 60° C. for long probes (e.g., greaterthan 50 nucleotides). Stringent conditions may also be achieved with theaddition of destabilizing agents such as formamide or Denhardt's.Exemplary low stringency conditions include hybridization with a buffersolution of 30 to 35% formamide, 1 M NaCl, 1% SDS (sodium dodecylsulphate) at 37° C., and a wash in 1× to 2×SSC (20×SSC=3.0 M NaCl/0.3 Mtrisodium citrate) at 50 to 55° C. Exemplary moderate stringencyconditions include hybridization in 40 to 45% formamide, 1 M NaCl, 1%SDS at 37° C., and a wash in 0.5× to 1×SSC at 55 to 60° C. Exemplaryhigh stringency conditions include hybridization in 50% formamide, 1 MNaCl, 1% SDS at 37° C. and a wash in 0.1×SSC at 60 to 65° C. Specificityis typically the function of post-hybridization washes, the criticalfactors being the ionic strength and temperature of the final washsolution. For DNA-DNA hybrids, the T_(m) can be approximated from theequation of Meinkoth and Wahl, (1984) Anal. Biochem., 138:267-84:T_(m)=81.5° C.+16.6 (log M)+0.41 (% GC)−0.61 (% form)−500/L; where M isthe molarity of monovalent cations, % GC is the percentage of guanosineand cytosine nucleotides in the DNA, % form is the percentage offormamide in the hybridization solution, and L is the length of thehybrid in base pairs. The T_(m) is the temperature (under defined ionicstrength and pH) at which 50% of a complementary target sequencehybridizes to a perfectly matched probe. T_(m) is reduced by about 1° C.for each 1% of mismatching; thus, T_(m), hybridization and/or washconditions can be adjusted to hybridize to sequences of the desiredidentity. For example, if sequences with ≧90% identity are sought, theT_(m) can be decreased 10° C. Generally, stringent conditions areselected to be about 5° C. lower than the thermal melting point (T_(m))for the specific sequence and its complement at a defined ionic strengthand pH. However, severely stringent conditions can utilize ahybridization and/or wash at 1, 2, 3 or 4° C. lower than the thermalmelting point (T_(m)); moderately stringent conditions can utilize ahybridization and/or wash at 6, 7, 8, 9 or 10° C. lower than the thermalmelting point (T_(m)); low stringency conditions can utilize ahybridization and/or wash at 11, 12, 13, 14, 15 or 20° C. lower than thethermal melting point (T_(m)). Using the equation, hybridization andwash compositions, and desired T_(m), those of ordinary skill willunderstand that variations in the stringency of hybridization and/orwash solutions are inherently described. If the desired degree ofmismatching results in a T_(m) of less than 45° C. (aqueous solution) or32° C. (formamide solution) it is preferred to increase the SSCconcentration so that a higher temperature can be used. An extensiveguide to the hybridization of nucleic acids is found in Tijssen,LABORATORY TECHNIQUES IN BIOCHEMISTRY AND MOLECULARBIOLOGY—HYBRIDIZATION WITH NUCLEIC ACID PROBES, part I, chapter 2,“Overview of principles of hybridization and the strategy of nucleicacid probe assays,” Elsevier, New York (1993); and CURRENT PROTOCOLS INMOLECULAR BIOLOGY, chapter 2, Ausubel, et al., eds, Greene Publishingand Wiley-Interscience, New York (1995). Unless otherwise stated, in thepresent application high stringency is defined as hybridization in4×SSC, 5×Denhardt's (5 g Ficoll, 5 g polyvinypyrrolidone, 5 g bovineserum albumin in 500 ml of water), 0.1 mg/ml boiled salmon sperm DNA,and 25 mM Na phosphate at 65° C., and a wash in 0.1×SSC, 0.1% SDS at 65°C.

As used herein, “transgenic plant” includes reference to a plant, whichcomprises within its genome a heterologous polynucleotide. Generally,the heterologous polynucleotide is stably integrated within the genomesuch that the polynucleotide is passed on to successive generations. Theheterologous polynucleotide may be integrated into the genome alone oras part of a recombinant expression cassette. “Transgenic” is usedherein to include any cell, cell line, callus, tissue, plant part orplant, the genotype of which has been altered by the presence ofheterologous nucleic acid including those transgenics initially soaltered as well as those created by sexual crosses or asexualpropagation from the initial transgenic. The term “transgenic” as usedherein does not encompass the alteration of the genome (chromosomal orextra-chromosomal) by conventional plant breeding methods or bynaturally occurring events such as random cross-fertilization,non-recombinant viral infection, non-recombinant bacterialtransformation, non-recombinant transposition or spontaneous mutation.

As used herein, “vector” includes reference to a nucleic acid used intransfection of a host cell and into which can be inserted apolynucleotide. Vectors are often replicons. Expression vectors permittranscription of a nucleic acid inserted therein.

The following terms are used to describe the sequence relationshipsbetween two or more nucleic acids or polynucleotides or polypeptides:(a) “reference sequence,” (b) “comparison window,” (c) “sequenceidentity,” (d) “percentage of sequence identity” and (e) “substantialidentity.”

As used herein, “reference sequence” is a defined sequence used as abasis for sequence comparison. A reference sequence may be a subset orthe entirety of a specified sequence; for example, as a segment of afull-length cDNA or gene sequence or the complete cDNA or gene sequence.

As used herein, “comparison window” means includes reference to acontiguous and specified segment of a polynucleotide sequence, whereinthe polynucleotide sequence may be compared to a reference sequence andwherein the portion of the polynucleotide sequence in the comparisonwindow may comprise additions or deletions (i.e., gaps) compared to thereference sequence (which does not comprise additions or deletions) foroptimal alignment of the two sequences. Generally, the comparison windowis at least 20 contiguous nucleotides in length, and optionally can be30, 40, 50, 100 or longer. Those of skill in the art understand that toavoid a high similarity to a reference sequence due to inclusion of gapsin the polynucleotide sequence a gap penalty is typically introduced andis subtracted from the number of matches.

Methods of alignment of nucleotide and amino acid sequences forcomparison are well known in the art. The local homology algorithm(BESTFIT) of Smith and Waterman, (1981) Adv. Appl. Math 2:482, mayconduct optimal alignment of sequences for comparison; by the homologyalignment algorithm (GAP) of Needleman and Wunsch, (1970) J. Mol. Biol.48:443-53; by the search for similarity method (Tfasta and Fasta) ofPearson and Lipman, (1988) Proc. Natl. Acad. Sci. USA 85:2444; bycomputerized implementations of these algorithms, including, but notlimited to: CLUSTAL in the PC/Gene program by Intelligenetics, MountainView, Calif., GAP, BESTFIT, BLAST, FASTA and TFASTA in the WisconsinGenetics Software Package®, Version 8 (available from Genetics ComputerGroup (GCG® programs (Accelrys, Inc., San Diego, Calif.).). The CLUSTALprogram is well described by Higgins and Sharp, (1988) Gene 73:237-44;Higgins and Sharp, (1989) CABIOS 5:151-3; Corpet, et al., (1988) NucleicAcids Res. 16:10881-90; Huang, et al., (1992) Computer Applications inthe Biosciences 8:155-65, and Pearson, et al., (1994) Meth. Mol. Biol.24:307-31. The preferred program to use for optimal global alignment ofmultiple sequences is PileUp (Feng and Doolittle, (1987) J. Mol. Evol.,25:351-60 which is similar to the method described by Higgins and Sharp,(1989) CABIOS 5:151-53 and hereby incorporated by reference). The BLASTfamily of programs which can be used for database similarity searchesincludes: BLASTN for nucleotide query sequences against nucleotidedatabase sequences; BLASTX for nucleotide query sequences againstprotein database sequences; BLASTP for protein query sequences againstprotein database sequences; TBLASTN for protein query sequences againstnucleotide database sequences and TBLASTX for nucleotide query sequencesagainst nucleotide database sequences. See, CURRENT PROTOCOLS INMOLECULAR BIOLOGY, Chapter 19, Ausubel, et al., eds., Greene Publishingand Wiley-Interscience, New York (1995).

GAP uses the algorithm of Needleman and Wunsch, supra, to find thealignment of two complete sequences that maximizes the number of matchesand minimizes the number of gaps. GAP considers all possible alignmentsand gap positions and creates the alignment with the largest number ofmatched bases and the fewest gaps. It allows for the provision of a gapcreation penalty and a gap extension penalty in units of matched bases.GAP must make a profit of gap creation penalty number of matches foreach gap it inserts. If a gap extension penalty greater than zero ischosen, GAP must, in addition, make a profit for each gap inserted ofthe length of the gap times the gap extension penalty. Default gapcreation penalty values and gap extension penalty values in Version 10of the Wisconsin Genetics Software Package® are 8 and 2, respectively.The gap creation and gap extension penalties can be expressed as aninteger selected from the group of integers consisting of from 0 to 100.Thus, for example, the gap creation and gap extension penalties can be0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, 40, 50 or greater.

GAP presents one member of the family of best alignments. There may bemany members of this family, but no other member has a better quality.GAP displays four figures of merit for alignments: Quality, Ratio,Identity and Similarity. The Quality is the metric maximized in order toalign the sequences. Ratio is the quality divided by the number of basesin the shorter segment. Percent Identity is the percent of the symbolsthat actually match. Percent Similarity is the percent of the symbolsthat are similar. Symbols that are across from gaps are ignored. Asimilarity is scored when the scoring matrix value for a pair of symbolsis greater than or equal to 0.50, the similarity threshold. The scoringmatrix used in Version 10 of the Wisconsin Genetics Software Package® isBLOSUM62 (see, Henikoff and Henikoff, (1989) Proc. Natl. Acad. Sci. USA89:10915).

Unless otherwise stated, sequence identity/similarity values providedherein refer to the value obtained using the BLAST 2.0 suite of programsusing default parameters (Altschul, et al., (1997) Nucleic Acids Res.25:3389-402).

As those of ordinary skill in the art will understand, BLAST searchesassume that proteins can be modeled as random sequences. However, manyreal proteins comprise regions of nonrandom sequences, which may behomopolymeric tracts, short-period repeats, or regions enriched in oneor more amino acids. Such low-complexity regions may be aligned betweenunrelated proteins even though other regions of the protein are entirelydissimilar. A number of low-complexity filter programs can be employedto reduce such low-complexity alignments. For example, the SEG (Wootenand Federhen, (1993) Comput. Chem. 17:149-63) and XNU (Claverie andStates, (1993) Comput. Chem. 17:191-201) low-complexity filters can beemployed alone or in combination.

As used herein, “sequence identity” or “identity” in the context of twonucleic acid or polypeptide sequences includes reference to the residuesin the two sequences, which are the same when aligned for maximumcorrespondence over a specified comparison window. When percentage ofsequence identity is used in reference to proteins it is recognized thatresidue positions which are not identical often differ by conservativeamino acid substitutions, where amino acid residues are substituted forother amino acid residues with similar chemical properties (e.g., chargeor hydrophobicity) and therefore do not change the functional propertiesof the molecule. Where sequences differ in conservative substitutions,the percent sequence identity may be adjusted upwards to correct for theconservative nature of the substitution. Sequences, which differ by suchconservative substitutions, are said to have “sequence similarity” or“similarity.” Means for making this adjustment are well known to thoseof skill in the art. Typically this involves scoring a conservativesubstitution as a partial rather than a full mismatch, therebyincreasing the percentage sequence identity. Thus, for example, where anidentical amino acid is given a score of 1 and a non-conservativesubstitution is given a score of zero, a conservative substitution isgiven a score between zero and 1. The scoring of conservativesubstitutions is calculated, e.g., according to the algorithm of Meyersand Miller, (1988) Computer Applic. Biol. Sci. 4:11-17, e.g., asimplemented in the program PC/GENE (Intelligenetics, Mountain View,Calif., USA).

As used herein, “percentage of sequence identity” means the valuedetermined by comparing two optimally aligned sequences over acomparison window, wherein the portion of the polynucleotide sequence inthe comparison window may comprise additions or deletions (i.e., gaps)as compared to the reference sequence (which does not comprise additionsor deletions) for optimal alignment of the two sequences. The percentageis calculated by determining the number of positions at which theidentical nucleic acid base or amino acid residue occurs in bothsequences to yield the number of matched positions, dividing the numberof matched positions by the total number of positions in the window ofcomparison and multiplying the result by 100 to yield the percentage ofsequence identity.

The term “substantial identity” of polynucleotide sequences means that apolynucleotide comprises a sequence that has between 50-100% sequenceidentity, preferably at least 50% sequence identity, preferably at least60% sequence identity, preferably at least 70%, more preferably at least80%, more preferably at least 90% and most preferably at least 95%,compared to a reference sequence using one of the alignment programsdescribed using standard parameters. One of skill will recognize thatthese values can be appropriately adjusted to determine correspondingidentity of proteins encoded by two nucleotide sequences by taking intoaccount codon degeneracy, amino acid similarity, reading framepositioning and the like. Substantial identity of amino acid sequencesfor these purposes normally means sequence identity of between 55-100%,preferably at least 55%, preferably at least 60%, more preferably atleast 70%, 80%, 90% and most preferably at least 95%.

Another indication that nucleotide sequences are substantially identicalis if two molecules hybridize to each other under stringent conditions.The degeneracy of the genetic code allows for many amino acidssubstitutions that lead to variety in the nucleotide sequence that codefor the same amino acid, hence it is possible that the DNA sequencecould code for the same polypeptide but not hybridize to each otherunder stringent conditions. This may occur, e.g., when a copy of anucleic acid is created using the maximum codon degeneracy permitted bythe genetic code. One indication that two nucleic acid sequences aresubstantially identical is that the polypeptide, which the first nucleicacid encodes, is immunologically cross reactive with the polypeptideencoded by the second nucleic acid.

The terms “substantial identity” in the context of a peptide indicatesthat a peptide comprises a sequence with between 55-100% sequenceidentity to a reference sequence preferably at least 55% sequenceidentity, preferably 60% preferably 70%, more preferably 80%, mostpreferably at least 90% or 95% sequence identity to the referencesequence over a specified comparison window. Preferably, optimalalignment is conducted using the homology alignment algorithm ofNeedleman and Wunsch, supra. An indication that two peptide sequencesare substantially identical is that one peptide is immunologicallyreactive with antibodies raised against the second peptide. Thus, apeptide is substantially identical to a second peptide, for example,where the two peptides differ only by a conservative substitution. Inaddition, a peptide can be substantially identical to a second peptidewhen they differ by a non-conservative change if the epitope that theantibody recognizes is substantially identical. Peptides, which are“substantially similar” share sequences as, noted above except thatresidue positions, which are not identical, may differ by conservativeamino acid changes.

The invention discloses AMT polynucleotides and polypeptides. The novelnucleotides and proteins of the invention have an expression patternwhich indicates that they regulate ammonium transport and thus play animportant role in plant development. The polynucleotides are expressedin various plant tissues. The polynucleotides and polypeptides thusprovide an opportunity to manipulate plant development to alter seed andvegetative tissue development, timing or composition. This may be usedto create a plant with altered N composition in source and sink.

Nucleic Acids

The present invention provides, inter alia, isolated nucleic acids ofRNA, DNA and analogs and/or chimeras thereof, comprising an amtpolynucleotide.

The present invention also includes polynucleotides optimized forexpression in different organisms. For example, for expression of thepolynucleotide in a maize plant, the sequence can be altered to accountfor specific codon preferences and to alter GC content as according toMurray, et al., supra. Maize codon usage for 28 genes from maize plantsis listed in Table 4 of Murray et al., supra.

The AMT nucleic acids of the present invention comprise isolated AMTpolynucleotides which are inclusive of:

-   -   (a) a polynucleotide encoding an AMT polypeptide and        conservatively modified and polymorphic variants thereof;    -   (b) a polynucleotide having at least 70% sequence identity with        polynucleotides of (a) or (b);    -   (c) complementary sequences of polynucleotides of (a) or (b).

Construction of Nucleic Acids

The isolated nucleic acids of the present invention can be made using(a) standard recombinant methods, (b) synthetic techniques, orcombinations thereof. In some embodiments, the polynucleotides of thepresent invention will be cloned, amplified, or otherwise constructedfrom a fungus or bacteria.

The nucleic acids may conveniently comprise sequences in addition to apolynucleotide of the present invention. For example, a multi-cloningsite comprising one or more endonuclease restriction sites may beinserted into the nucleic acid to aid in isolation of thepolynucleotide. Also, translatable sequences may be inserted to aid inthe isolation of the translated polynucleotide of the present invention.For example, a hexa-histidine marker sequence provides a convenientmeans to purify the proteins of the present invention. The nucleic acidof the present invention—excluding the polynucleotide sequence—isoptionally a vector, adapter or linker for cloning and/or expression ofa polynucleotide of the present invention. Additional sequences may beadded to such cloning and/or expression sequences to optimize theirfunction in cloning and/or expression, to aid in isolation of thepolynucleotide, or to improve the introduction of the polynucleotideinto a cell. Typically, the length of a nucleic acid of the presentinvention less the length of its polynucleotide of the present inventionis less than 20 kilobase pairs, often less than 15 kb and frequentlyless than 10 kb. Use of cloning vectors, expression vectors, adaptersand linkers is well known in the art. Exemplary nucleic acids includesuch vectors as: M13, lambda ZAP Express, lambda ZAP II, lambda gt10,lambda gt11, pBK-CMV, pBK-RSV, pBluescript II, lambda DASH II, lambdaEMBL 3, lambda EMBL 4, pWE15, SuperCos 1, SurfZap, Uni-ZAP, pBC, pBS+/−,pSG5, pBK, pCR-Script, pET, pSPUTK, p3′SS, pGEM, pSK+/−, pGEX, pSPORTIand II, pOPRSVI CAT, pOPI3 CAT, pXT1, pSG5, pPbac, pMbac, pMC1neo,pOG44, pOG45, pFRTβGAL, pNEOβGAL, pRS403, pRS404, pRS405, pRS406,pRS413, pRS414, pRS415, pRS416, lambda MOSSlox and lambda MOSElox.Optional vectors for the present invention, include but are not limitedto, lambda ZAP II and pGEX. For a description of various nucleic acidssee, e.g., Stratagene Cloning Systems, Catalogs 1995, 1996, 1997 (LaJolla, Calif.); and, Amersham Life Sciences, Inc, Catalog '97 (ArlingtonHeights, Ill.).

Synthetic Methods for Constructing Nucleic Acids

The isolated nucleic acids of the present invention can also be preparedby direct chemical synthesis by methods such as the phosphotriestermethod of Narang, et al., (1979) Meth. Enzymol. 68:90-9; thephosphodiester method of Brown, et al., (1979) Meth. Enzymol. 68:109-51;the diethylphosphoramidite method of Beaucage, et al., (1981) Tetra.Letts. 22(20):1859-62; the solid phase phosphoramidite triester methoddescribed by Beaucage, et al., supra, e.g., using an automatedsynthesizer, e.g., as described in Needham-VanDevanter, et al., (1984)Nucleic Acids Res. 12:6159-68 and the solid support method of U.S. Pat.No. 4,458,066. Chemical synthesis generally produces a single strandedoligonucleotide. This may be converted into double stranded DNA byhybridization with a complementary sequence or by polymerization with aDNA polymerase using the single strand as a template. One of skill willrecognize that while chemical synthesis of DNA is limited to sequencesof about 100 bases, longer sequences may be obtained by the ligation ofshorter sequences.

UTRs and Codon Preference

In general, translational efficiency has been found to be regulated byspecific sequence elements in the 5′ non-coding or untranslated region(5′ UTR) of the RNA. Positive sequence motifs include translationalinitiation consensus sequences (Kozak, (1987) Nucleic Acids Res.15:8125) and the 5<G>7 methyl GpppG RNA cap structure (Drummond, et al.,(1985) Nucleic Acids Res. 13:7375). Negative elements include stableintramolecular 5′ UTR stem-loop structures (Muesing, et al., (1987) Cell48:691) and AUG sequences or short open reading frames preceded by anappropriate AUG in the 5′ UTR (Kozak, supra, Rao, et al., (1988) Mol.and Cell. Biol. 8:284). Accordingly, the present invention provides 5′and/or 3′ UTR regions for modulation of translation of heterologouscoding sequences.

Further, the polypeptide-encoding segments of the polynucleotides of thepresent invention can be modified to alter codon usage. Altered codonusage can be employed to alter translational efficiency and/or tooptimize the coding sequence for expression in a desired host or tooptimize the codon usage in a heterologous sequence for expression inmaize. Codon usage in the coding regions of the polynucleotides of thepresent invention can be analyzed statistically using commerciallyavailable software packages such as “Codon Preference” available fromthe University of Wisconsin Genetics Computer Group. See, Devereaux, etal., (1984) Nucleic Acids Res. 12:387-395; or MacVector 4.1 (EastmanKodak Co., New Haven, Conn.). Thus, the present invention provides acodon usage frequency characteristic of the coding region of at leastone of the polynucleotides of the present invention. The number ofpolynucleotides (3 nucleotides per amino acid) that can be used todetermine a codon usage frequency can be any integer from 3 to thenumber of polynucleotides of the present invention as provided herein.Optionally, the polynucleotides will be full-length sequences. Anexemplary number of sequences for statistical analysis can be at least1, 5, 10, 20, 50 or 100.

Sequence Shuffling

The present invention provides methods for sequence shuffling usingpolynucleotides of the present invention, and compositions resultingtherefrom. Sequence shuffling is described in PCT Publication Number1996/19256. See also, Zhang, et al., (1997) Proc. Natl. Acad. Sci. USA94:4504-9 and Zhao, et al., (1998) Nature Biotech 16:258-61. Generally,sequence shuffling provides a means for generating libraries ofpolynucleotides having a desired characteristic, which can be selectedor screened for. Libraries of recombinant polynucleotides are generatedfrom a population of related sequence polynucleotides, which comprisesequence regions, which have substantial sequence identity and can behomologously recombined in vitro or in vivo. The population ofsequence-recombined polynucleotides comprises a subpopulation ofpolynucleotides which possess desired or advantageous characteristicsand which can be selected by a suitable selection or screening method.The characteristics can be any property or attribute capable of beingselected for or detected in a screening system, and may includeproperties of: an encoded protein, a transcriptional element, a sequencecontrolling transcription, RNA processing, RNA stability, chromatinconformation, translation or other expression property of a gene ortransgene, a replicative element, a protein-binding element, or thelike, such as any feature which confers a selectable or detectableproperty. In some embodiments, the selected characteristic will be analtered K_(m) and/or K_(cat) over the wild-type protein as providedherein. In other embodiments, a protein or polynucleotide generated fromsequence shuffling will have a ligand binding affinity greater than thenon-shuffled wild-type polynucleotide. In yet other embodiments, aprotein or polynucleotide generated from sequence shuffling will have analtered pH optimum as compared to the non-shuffled wild-typepolynucleotide. The increase in such properties can be at least 110%,120%, 130%, 140% or greater than 150% of the wild-type value.

Recombinant Expression Cassettes

The present invention further provides recombinant expression cassettescomprising a nucleic acid of the present invention. A nucleic acidsequence coding for the desired polynucleotide of the present invention,for example a cDNA or a genomic sequence encoding a polypeptide longenough to code for an active protein of the present invention, can beused to construct a recombinant expression cassette which can beintroduced into the desired host cell. A recombinant expression cassettewill typically comprise a polynucleotide of the present inventionoperably linked to transcriptional initiation regulatory sequences whichwill direct the transcription of the polynucleotide in the intended hostcell, such as tissues of a transformed plant.

For example, plant expression vectors may include (1) a cloned plantgene under the transcriptional control of 5′ and 3′ regulatory sequencesand (2) a dominant selectable marker. Such plant expression vectors mayalso contain, if desired, a promoter regulatory region (e.g., oneconferring inducible or constitutive, environmentally- ordevelopmentally-regulated or cell- or tissue-specific/selectiveexpression), a transcription initiation start site, a ribosome bindingsite, an RNA processing signal, a transcription termination site and/ora polyadenylation signal.

A plant promoter fragment can be employed which will direct expressionof a polynucleotide of the present invention in all tissues of aregenerated plant. Such promoters are referred to herein as“constitutive” promoters and are active under most environmentalconditions and states of development or cell differentiation. Examplesof constitutive promoters include the 1′- or 2′-promoter derived fromT-DNA of Agrobacterium tumefaciens, the Smas promoter, the cinnamylalcohol dehydrogenase promoter (U.S. Pat. No. 5,683,439), the Nospromoter, the rubisco promoter, the GRP1-8 promoter, the 35S promoterfrom cauliflower mosaic virus (CaMV), as described in Odell, et al.,(1985) Nature 313:810-2; rice actin (McElroy, et al., (1990) Plant Cell163-171); ubiquitin (Christensen, et al., (1992) Plant Mol. Biol.12:619-632 and Christensen, et al., (1992) Plant Mol. Biol. 18:675-89);pEMU (Last, et al., (1991) Theor. Appl. Genet. 81:581-8); MAS (Velten,et al., (1984) EMBO J. 3:2723-30) and maize H3 histone (Lepetit, et al.,(1992) Mol. Gen. Genet. 231:276-85 and Atanassvoa, et al., (1992) PlantJournal 2(3):291-300); ALS promoter, as described in PCT ApplicationNumber WO 1996/30530 and other transcription initiation regions fromvarious plant genes known to those of skill. For the present inventionubiquitin is the preferred promoter for expression in monocot plants.

Alternatively, the plant promoter can direct expression of apolynucleotide of the present invention in a specific tissue or may beotherwise under more precise environmental or developmental control.Such promoters are referred to here as “inducible” promoters.Environmental conditions that may effect transcription by induciblepromoters include pathogen attack, anaerobic conditions or the presenceof light. Examples of inducible promoters are the Adh1 promoter, whichis inducible by hypoxia or cold stress, the Hsp70 promoter, which isinducible by heat stress, and the PPDK promoter, which is inducible bylight.

Examples of promoters under developmental control include promoters thatinitiate transcription only, or preferentially, in certain tissues, suchas leaves, roots, fruit, seeds or flowers. The operation of a promotermay also vary depending on its location in the genome. Thus, aninducible promoter may become fully or partially constitutive in certainlocations.

If polypeptide expression is desired, it is generally desirable toinclude a polyadenylation region at the 3′-end of a polynucleotidecoding region. The polyadenylation region can be derived from a varietyof plant genes, or from T-DNA. The 3′ end sequence to be added can bederived from, for example, the nopaline synthase or octopine synthasegenes or alternatively from another plant gene or less preferably fromany other eukaryotic gene. Examples of such regulatory elements include,but are not limited to, 3′ termination and/or polyadenylation regionssuch as those of the Agrobacterium tumefaciens nopaline synthase (nos)gene (Bevan, et al., (1983) Nucleic Acids Res. 12:369-85); the potatoproteinase inhibitor II (PI NII) gene (Keil, et al., (1986) NucleicAcids Res. 14:5641-50 and An, et al., (1989) Plant Cell 1:115-22) andthe CaMV 19S gene (Mogen, et al., (1990) Plant Cell 2:1261-72).

An intron sequence can be added to the 5′ untranslated region or thecoding sequence of the partial coding sequence to increase the amount ofthe mature message that accumulates in the cytosol. Inclusion of aspliceable intron in the transcription unit in both plant and animalexpression constructs has been shown to increase gene expression at boththe mRNA and protein levels up to 1000-fold (Buchman and Berg, (1988)Mol. Cell Biol. 8:4395-4405; Callis, et al., (1987) Genes Dev.1:1183-200). Such intron enhancement of gene expression is typicallygreatest when placed near the 5′ end of the transcription unit. Use ofmaize introns Adh1-S intron 1, 2, and 6, the Bronze-1 intron are knownin the art. See generally, THE MAIZE HANDBOOK, Chapter 116, Freeling andWalbot, eds., Springer, New York (1994).

Plant signal sequences, including, but not limited to, signal-peptideencoding DNA/RNA sequences which target proteins to the extracellularmatrix of the plant cell (Dratewka-Kos, et al., (1989) J. Biol. Chem.264:4896-900), such as the Nicotiana plumbaginifolia extension gene(DeLoose, et al., (1991) Gene 99:95-100); signal peptides which targetproteins to the vacuole, such as the sweet potato sporamin gene(Matsuka, et al., (1991) Proc. Natl. Acad. Sci. USA 88:834) and thebarley lectin gene (Wilkins, et al., (1990) Plant Cell, 2:301-13);signal peptides which cause proteins to be secreted, such as that ofPRIb (Lind, et al., (1992) Plant Mol. Biol. 18:47-53) or the barleyalpha amylase (BAA) (Rahmatullah, et al., (1989) Plant Mol. Biol.12:119, and hereby incorporated by reference) or signal peptides whichtarget proteins to the plastids such as that of rapeseed enoyl-Acpreductase (Verwaert, et al., (1994) Plant Mol. Biol. 26:189-202) areuseful in the invention. The barley alpha amylase signal sequence fusedto the AMT polynucleotide is the preferred construct for expression inmaize for the present invention.

The vector comprising the sequences from a polynucleotide of the presentinvention will typically comprise a marker gene, which confers aselectable phenotype on plant cells. Usually, the selectable marker genewill encode antibiotic resistance, with suitable genes including genescoding for resistance to the antibiotic spectinomycin (e.g., the aadagene), the streptomycin phosphotransferase (SPT) gene coding forstreptomycin resistance, the neomycin phosphotransferase (NPTII) geneencoding kanamycin or geneticin resistance, the hygromycinphosphotransferase (HPT) gene coding for hygromycin resistance, genescoding for resistance to herbicides which act to inhibit the action ofacetolactate synthase (ALS), in particular the sulfonylurea-typeherbicides (e.g., the acetolactate synthase (ALS) gene containingmutations leading to such resistance in particular the S4 and/or Hramutations), genes coding for resistance to herbicides which act toinhibit action of glutamine synthase, such as phosphinothricin or basta(e.g., the bar gene), or other such genes known in the art. The bar geneencodes resistance to the herbicide basta and the ALS gene encodesresistance to the herbicide chlorsulfuron.

Typical vectors useful for expression of genes in higher plants are wellknown in the art and include vectors derived from the tumor-inducing(Ti) plasmid of Agrobacterium tumefaciens described by Rogers, et al.,(1987) Meth. Enzymol 153:253-77. These vectors are plant integratingvectors in that on transformation, the vectors integrate a portion ofvector DNA into the genome of the host plant. Exemplary A. tumefaciensvectors useful herein are plasmids pKYLX6 and pKYLX7 of Schardl, et al.,(1987) Gene 61:1-11 and Berger, et al., (1989) Proc. Natl. Acad. Sci.USA, 86:8402-6. Another useful vector herein is plasmid pBI101.2 that isavailable from CLONTECH Laboratories, Inc. (Palo Alto, Calif.).

Expression of Proteins in Host Cells

Using the nucleic acids of the present invention, one may express aprotein of the present invention in a recombinantly engineered cell suchas bacteria, yeast, insect, mammalian or preferably plant cells. Thecells produce the protein in a non-natural condition (e.g., in quantity,composition, location and/or time), because they have been geneticallyaltered through human intervention to do so.

It is expected that those of skill in the art are knowledgeable in thenumerous expression systems available for expression of a nucleic acidencoding a protein of the present invention. No attempt to describe indetail the various methods known for the expression of proteins inprokaryotes or eukaryotes will be made.

In brief summary, the expression of isolated nucleic acids encoding aprotein of the present invention will typically be achieved by operablylinking, for example, the DNA or cDNA to a promoter (which is eitherconstitutive or inducible), followed by incorporation into an expressionvector. The vectors can be suitable for replication and integration ineither prokaryotes or eukaryotes. Typical expression vectors containtranscription and translation terminators, initiation sequences andpromoters useful for regulation of the expression of the DNA encoding aprotein of the present invention. To obtain high level expression of acloned gene, it is desirable to construct expression vectors whichcontain, at the minimum, a strong promoter, such as ubiquitin, to directtranscription, a ribosome binding site for translational initiation anda transcription/translation terminator. Constitutive promoters areclassified as providing for a range of constitutive expression. Thus,some are weak constitutive promoters, and others are strong constitutivepromoters. Generally, by “weak promoter” is intended a promoter thatdrives expression of a coding sequence at a low level. By “low level” isintended at levels of about 1/10,000 transcripts to about 1/100,000transcripts to about 1/500,000 transcripts. Conversely, a “strongpromoter” drives expression of a coding sequence at a “high level,” orabout 1/10 transcripts to about 1/100 transcripts to about 1/1,000transcripts.

One of skill would recognize that modifications could be made to aprotein of the present invention without diminishing its biologicalactivity. Some modifications may be made to facilitate the cloning,expression or incorporation of the targeting molecule into a fusionprotein. Such modifications are well known to those of skill in the artand include, for example, a methionine added at the amino terminus toprovide an initiation site or additional amino acids (e.g., poly His)placed on either terminus to create conveniently located restrictionsites or termination codons or purification sequences.

Expression in Prokaryotes

Prokaryotic cells may be used as hosts for expression. Prokaryotes mostfrequently are represented by various strains of E. coli; however, othermicrobial strains may also be used. Commonly used prokaryotic controlsequences which are defined herein to include promoters fortranscription initiation, optionally with an operator, along withribosome binding site sequences, include such commonly used promoters asthe beta lactamase (penicillinase) and lactose (lac) promoter systems(Chang, et al., (1977) Nature 198:1056), the tryptophan (trp) promotersystem (Goeddel, et al., (1980) Nucleic Acids Res. 8:4057) and thelambda derived P L promoter and N-gene ribosome binding site (Shimatake,et al., (1981) Nature 292:128). The inclusion of selection markers inDNA vectors transfected in E. coli is also useful. Examples of suchmarkers include genes specifying resistance to ampicillin, tetracyclineor chloramphenicol.

The vector is selected to allow introduction of the gene of interestinto the appropriate host cell. Bacterial vectors are typically ofplasmid or phage origin. Appropriate bacterial cells are infected withphage vector particles or transfected with naked phage vector DNA. If aplasmid vector is used, the bacterial cells are transfected with theplasmid vector DNA. Expression systems for expressing a protein of thepresent invention are available using Bacillus sp. and Salmonella(Palva, et al., (1983) Gene 22:229-35; Mosbach, et al., (1983) Nature302:543-5). The pGEX-4T-1 plasmid vector from Pharmacia is the preferredE. coli expression vector for the present invention.

Expression in Eukaryotes

A variety of eukaryotic expression systems such as yeast, insect celllines, plant and mammalian cells, are known to those of skill in theart. As explained briefly below, the present invention can be expressedin these eukaryotic systems. In some embodiments,transformed/transfected plant cells, as discussed infra, are employed asexpression systems for production of the proteins of the instantinvention.

Synthesis of heterologous proteins in yeast is well known. Sherman, etal., METHODS IN YEAST GENETICS, Cold Spring Harbor Laboratory (1982) isa well recognized work describing the various methods available toproduce the protein in yeast. Two widely utilized yeasts for productionof eukaryotic proteins are Saccharomyces cerevisiae and Pichia pastoris.Vectors, strains and protocols for expression in Saccharomyces andPichia are known in the art and available from commercial suppliers(e.g., Invitrogen). Suitable vectors usually have expression controlsequences, such as promoters, including 3-phosphoglycerate kinase oralcohol oxidase and an origin of replication, termination sequences andthe like as desired.

A protein of the present invention, once expressed, can be isolated fromyeast by lysing the cells and applying standard protein isolationtechniques to the lysates or the pellets. The monitoring of thepurification process can be accomplished by using Western blottechniques or radioimmunoassay of other standard immunoassay techniques.

The sequences encoding proteins of the present invention can also beligated to various expression vectors for use in transfecting cellcultures of, for instance, mammalian, insect or plant origin. Mammaliancell systems often will be in the form of monolayers of cells althoughmammalian cell suspensions may also be used. A number of suitable hostcell lines capable of expressing intact proteins have been developed inthe art, and include the HEK293, BHK21 and CHO cell lines. Expressionvectors for these cells can include expression control sequences, suchas an origin of replication, a promoter (e.g., the CMV promoter, a HSVtk promoter or pgk (phosphoglycerate kinase) promoter), an enhancer(Queen, et al., (1986) Immunol. Rev. 89:49) and necessary processinginformation sites, such as ribosome binding sites, RNA splice sites,polyadenylation sites (e.g., an SV40 large T Ag poly A addition site)and transcriptional terminator sequences. Other animal cells useful forproduction of proteins of the present invention are available, forinstance, from the American Type Culture Collection Catalogue of CellLines and Hybridomas (7^(th) ed., 1992).

Appropriate vectors for expressing proteins of the present invention ininsect cells are usually derived from the SF9 baculovirus. Suitableinsect cell lines include mosquito larvae, silkworm, armyworm, moth andDrosophila cell lines such as a Schneider cell line (see, e.g.,Schneider, (1987) J. Embryol. Exp. Morphol. 27:353-65).

As with yeast, when higher animal or plant host cells are employed,polyadenlyation or transcription terminator sequences are typicallyincorporated into the vector. An example of a terminator sequence is thepolyadenlyation sequence from the bovine growth hormone gene. Sequencesfor accurate splicing of the transcript may also be included. An exampleof a splicing sequence is the VP1 intron from SV40 (Sprague, et al.,(1983) J. Virol. 45:773-81). Additionally, gene sequences to controlreplication in the host cell may be incorporated into the vector such asthose found in bovine papilloma virus type-vectors (Saveria-Campo,“Bovine Papilloma Virus DNA a Eukaryotic Cloning Vector,” in DNACLONING: A PRACTICAL APPROACH, vol. II, Glover, ed., IRL Press,Arlington, Va., pp. 213-38 (1985)).

In addition, the gene for AMT placed in the appropriate plant expressionvector can be used to transform plant cells. The polypeptide can then beisolated from plant callus or the transformed cells can be used toregenerate transgenic plants. Such transgenic plants can be harvested,and the appropriate tissues (seed or leaves, for example) can besubjected to large scale protein extraction and purification techniques.

Plant Transformation Methods

Numerous methods for introducing foreign genes into plants are known andcan be used to insert an amt polynucleotide into a plant host, includingbiological and physical plant transformation protocols. See, e.g., Miki,et al., “Procedure for Introducing Foreign DNA into Plants,” in METHODSIN PLANT MOLECULAR BIOLOGY AND BIOTECHNOLOGY, Glick and Thompson, eds.,CRC Press, Inc., Boca Raton, pp. 67-88 (1993). The methods chosen varywith the host plant, and include chemical transfection methods such ascalcium phosphate, microorganism-mediated gene transfer such asAgrobacterium (Horsch, et al., (1985) Science 227:1229-31),electroporation, micro-injection, and biolistic bombardment.

Expression cassettes and vectors and in vitro culture methods for plantcell or tissue transformation and regeneration of plants are known andavailable. See, e.g., Gruber, et al., “Vectors for PlantTransformation,” in METHODS IN PLANT MOLECULAR BIOLOGY ANDBIOTECHNOLOGY, supra, pp. 89-119.

The isolated polynucleotides or polypeptides may be introduced into theplant by one or more techniques typically used for direct delivery intocells. Such protocols may vary depending on the type of organism, cell,plant or plant cell, i.e., monocot or dicot, targeted for genemodification. Suitable methods of transforming plant cells includemicroinjection (Crossway, et al., (1986) Biotechniques 4:320-334 andU.S. Pat. No. 6,300,543), electroporation (Riggs, et al., (1986) Proc.Natl. Acad. Sci. USA 83:5602-5606, direct gene transfer (Paszkowski, etal., (1984) EMBO J. 3:2717-2722) and ballistic particle acceleration(see, for example, Sanford, et al., U.S. Pat. No. 4,945,050; WO1991/10725 and McCabe, et al., (1988) Biotechnology 6:923-926). Alsosee, Tomes, et al., Direct DNA Transfer into Intact Plant Cells ViaMicroprojectile Bombardment. pp. 197-213 in Plant Cell, Tissue and OrganCulture, Fundamental Methods. eds. Gamborg and Phillips. Springer-VerlagBerlin Heidelberg New York, 1995; U.S. Pat. No. 5,736,369 (meristem);Weissinger, et al., (1988) Ann. Rev. Genet. 22:421-477; Sanford, et al.,(1987) Particulate Science and Technology 5:27-37 (onion); Christou, etal., (1988) Plant Physiol. 87:671-674 (soybean); Datta, et al., (1990)Biotechnology 8:736-740 (rice); Klein, et al., (1988) Proc. Natl. Acad.Sci. USA 85:4305-4309 (maize); Klein, et al., (1988) Biotechnology6:559-563 (maize); WO 1991/10725 (maize); Klein, et al., (1988) PlantPhysiol. 91:440-444 (maize); Fromm, et al., (1990) Biotechnology8:833-839 and Gordon-Kamm, et al., (1990) Plant Cell 2:603-618 (maize);Hooydaas-Van Slogteren and Hooykaas (1984) Nature (London) 311:763-764;Bytebier, et al., (1987) Proc. Natl. Acad. Sci. USA 84:5345-5349(Liliaceae); De Wet, et al., (1985) In The Experimental Manipulation ofOvule Tissues, ed. G. P. Chapman, et al., pp. 197-209 Longman, NY(pollen); Kaeppler, et al., (1990) Plant Cell Reports 9:415-418; andKaeppler, et al., (1992) Theor. Appl. Genet. 84:560-566(whisker-mediated transformation); U.S. Pat. No. 5,693,512 (sonication);D'Halluin, et al., (1992) Plant Cell 4:1495-1505 (electroporation); Li,et al., (1993) Plant Cell Reports 12:250-255 and Christou and Ford,(1995) Annals of Botany 75:407-413 (rice); Osjoda, et al., (1996) NatureBiotech. 14:745-750; Agrobacterium mediated maize transformation (U.S.Pat. No. 5,981,840); silicon carbide whisker methods (Frame, et al.,(1994) Plant J. 6:941-948); laser methods (Guo, et al., (1995)Physiologia Plantarum 93:19-24); sonication methods (Bao, et al., (1997)Ultrasound in Medicine & Biology 23:953-959; Finer and Finer, (2000)Lett Appl Microbiol. 30:406-10; Amoah, et al., (2001) J Exp Bot52:1135-42); polyethylene glycol methods (Krens, et al., (1982) Nature296:72-77); protoplasts of monocot and dicot cells can be transformedusing electroporation (Fromm, et al., (1985) Proc. Natl. Acad. Sci. USA82:5824-5828) and microinjection (Crossway, et al., (1986) Mol. Gen.Genet. 202:179-185), all of which are herein incorporated by reference.

Agrobacterium-Mediated Transformation

The most widely utilized method for introducing an expression vectorinto plants is based on the natural transformation system ofAgrobacterium. A. tumefaciens and A. rhizogenes are plant pathogenicsoil bacteria, which genetically transform plant cells. The Ti and Riplasmids of A. tumefaciens and A. rhizogenes, respectively, carry genesresponsible for genetic transformation of plants. See, e.g., Kado,(1991) Crit. Rev. Plant Sci. 10:1. Descriptions of the Agrobacteriumvector systems and methods for Agrobacterium-mediated gene transfer areprovided in Gruber, et al., supra; Miki, et al., supra and Moloney, etal., (1989) Plant Cell Reports 8:238.

Similarly, the gene can be inserted into the T-DNA region of a Ti or Riplasmid derived from A. tumefaciens or A. rhizogenes, respectively.Thus, expression cassettes can be constructed as above, using theseplasmids. Many control sequences are known which when coupled to aheterologous coding sequence and transformed into a host organism showfidelity in gene expression with respect to tissue/organ specificity ofthe original coding sequence. See, e.g., Benfey and Chua, (1989) Science244:174-81. Particularly suitable control sequences for use in theseplasmids are promoters for constitutive leaf-specific expression of thegene in the various target plants. Other useful control sequencesinclude a promoter and terminator from the nopaline synthase gene (NOS).The NOS promoter and terminator are present in the plasmid pARC2,available from the American Type Culture Collection and designated ATCC67238. If such a system is used, the virulence (vir) gene from eitherthe Ti or Ri plasmid must also be present, either along with the T-DNAportion, or via a binary system where the vir gene is present on aseparate vector. Such systems, vectors for use therein, and methods oftransforming plant cells are described in U.S. Pat. No. 4,658,082; USPatent Application Serial Number 913,914, filed Oct. 1, 1986, asreferenced in U.S. Pat. No. 5,262,306, issued Nov. 16, 1993 and Simpson,et al., (1986) Plant Mol. Biol. 6:403-15 (also referenced in the '306patent), all incorporated by reference in their entirety.

Once constructed, these plasmids can be placed into A. rhizogenes or A.tumefaciens and these vectors used to transform cells of plant species,which are ordinarily susceptible to Fusarium or Alternaria infection.Several other transgenic plants are also contemplated by the presentinvention including but not limited to soybean, corn, sorghum, alfalfa,rice, clover, cabbage, banana, coffee, celery, tobacco, cowpea, cotton,melon, switchgrass, myscanthus, triticale and pepper. The selection ofeither A. tumefaciens or A. rhizogenes will depend on the plant beingtransformed thereby. In general A. tumefaciens is the preferred organismfor transformation. Most dicotyledonous plants, some gymnosperms and afew monocotyledonous plants (e.g., certain members of the Liliales andArales) are susceptible to infection with A. tumefaciens. A. rhizogenesalso has a wide host range, embracing most dicots and some gymnosperms,which includes members of the Leguminosae, Compositae andChenopodiaceae. Monocot plants can now be transformed with some success.EP Patent Application Number 604 662 A1 discloses a method fortransforming monocots using Agrobacterium. EP Application Number 672 752A1 discloses a method for transforming monocots with Agrobacterium usingthe scutellum of immature embryos. Ishida, et al., discuss a method fortransforming maize by exposing immature embryos to A. tumefaciens(Nature Biotechnology 14:745-50 (1996)).

Once transformed, these cells can be used to regenerate transgenicplants. For example, whole plants can be infected with these vectors bywounding the plant and then introducing the vector into the wound site.Any part of the plant can be wounded, including leaves, stems and roots.Alternatively, plant tissue, in the form of an explant, such ascotyledonary tissue or leaf disks, can be inoculated with these vectors,and cultured under conditions, which promote plant regeneration. Rootsor shoots transformed by inoculation of plant tissue with A. rhizogenesor A. tumefaciens, containing the gene coding for the fumonisindegradation enzyme, can be used as a source of plant tissue toregenerate fumonisin-resistant transgenic plants, either via somaticembryogenesis or organogenesis. Examples of such methods forregenerating plant tissue are disclosed in Shahin, (1985) Theor. Appl.Genet. 69:235-40; U.S. Pat. No. 4,658,082; Simpson, et al., supra and USPatent Application Serial Numbers 913,913 and 913,914, both filed Oct.1, 1986, as referenced in U.S. Pat. No. 5,262,306, issued Nov. 16, 1993,the entire disclosures therein incorporated herein by reference.

Direct Gene Transfer

Despite the fact that the host range for Agrobacterium-mediatedtransformation is broad, some major cereal crop species and gymnospermshave generally been recalcitrant to this mode of gene transfer, eventhough some success has recently been achieved in rice (Hiei, et al.,(1994) The Plant Journal 6:271-82). Several methods of planttransformation, collectively referred to as direct gene transfer, havebeen developed as an alternative to Agrobacterium-mediatedtransformation.

A generally applicable method of plant transformation ismicroprojectile-mediated transformation, where DNA is carried on thesurface of microprojectiles measuring about 1 to 4 μm. The expressionvector is introduced into plant tissues with a biolistic device thataccelerates the microprojectiles to speeds of 300 to 600 m/s which issufficient to penetrate the plant cell walls and membranes (Sanford, etal., (1987) Part. Sci. Technol. 5:27; Sanford, (1988) Trends Biotech6:299; Sanford, (1990) Physiol. Plant 79:206 and Klein, et al., (1992)Biotechnology 10:268).

Another method for physical delivery of DNA to plants is sonication oftarget cells as described in Zang, et al., (1991) BioTechnology 9:996.Alternatively, liposome or spheroplast fusions have been used tointroduce expression vectors into plants. See, e.g., Deshayes, et al.,(1985) EMBO J. 4:2731 and Christou, et al., (1987) Proc. Natl. Acad.Sci. USA 84:3962. Direct uptake of DNA into protoplasts using CaCl₂precipitation, polyvinyl alcohol, or poly-L-ornithine has also beenreported. See, e.g., Hain, et al., (1985) Mol. Gen. Genet. 199:161 andDraper, et al., (1982) Plant Cell Physiol. 23:451.

Electroporation of protoplasts and whole cells and tissues has also beendescribed. See, e.g., Donn, et al., (1990) in Abstracts of the VIIthInt'l. Congress on Plant Cell and Tissue Culture IAPTC, A2-38, p. 53;D'Halluin, et al., (1992) Plant Cell 4:1495-505 and Spencer, et al.,(1994) Plant Mol. Biol. 24:51-61.

Increasing the Activity and/or Level of an amt Polypeptide

Methods are provided to increase the activity and/or level of the AMTpolypeptide of the invention. An increase in the level and/or activityof the AMT polypeptide of the invention can be achieved by providing tothe plant an amt polypeptide. The AMT polypeptide can be provided byintroducing the amino acid sequence encoding the AMT polypeptide intothe plant, introducing into the plant a nucleotide sequence encoding anamt polypeptide or alternatively by modifying a genomic locus encodingthe AMT polypeptide of the invention.

As discussed elsewhere herein, many methods are known the art forproviding a polypeptide to a plant including, but not limited to, directintroduction of the polypeptide into the plant, introducing into theplant (transiently or stably) a polynucleotide construct encoding apolypeptide having AMT transporter activity. It is also recognized thatthe methods of the invention may employ a polynucleotide that is notcapable of directing, in the transformed plant, the expression of aprotein or an RNA. Thus, the level and/or activity of an amt polypeptidemay be increased by altering the gene encoding the AMT polypeptide orits promoter. See, e.g., Kmiec, U.S. Pat. No. 5,565,350; Zarling, etal., PCT/US93/03868. Therefore mutagenized plants that carry mutationsin AMT genes, where the mutations increase expression of the AMT gene orincrease the AMT transporter activity of the encoded AMT polypeptide areprovided.

Reducing the Activity and/or Level of an amt Polypeptide

Methods are provided to reduce or eliminate the activity of an amtpolypeptide of the invention by transforming a plant cell with anexpression cassette that expresses a polynucleotide that inhibits theexpression of the AMT polypeptide. The polynucleotide may inhibit theexpression of the AMT polypeptide directly, by preventing transcriptionor translation of the AMT messenger RNA, or indirectly, by encoding apolypeptide that inhibits the transcription or translation of an amtgene encoding an amt polypeptide. Methods for inhibiting or eliminatingthe expression of a gene in a plant are well known in the art, and anysuch method may be used in the present invention to inhibit theexpression of an amt polypeptide.

In accordance with the present invention, the expression of an amtpolypeptide is inhibited if the protein level of the AMT polypeptide isless than 70% of the protein level of the same AMT polypeptide in aplant that has not been genetically modified or mutagenized to inhibitthe expression of that AMT polypeptide. In particular embodiments of theinvention, the protein level of the AMT polypeptide in a modified plantaccording to the invention is less than 60%, less than 50%, less than40%, less than 30%, less than 20%, less than 10%, less than 5% or lessthan 2% of the protein level of the same AMT polypeptide in a plant thatis not a mutant or that has not been genetically modified to inhibit theexpression of that AMT polypeptide. The expression level of the AMTpolypeptide may be measured directly, for example, by assaying for thelevel of AMT polypeptide expressed in the plant cell or plant, orindirectly, for example, by measuring the AMT transporter activity ofthe AMT polypeptide in the plant cell or plant, or by measuring the AMTin the plant. Methods for performing such assays are described elsewhereherein.

In other embodiments of the invention, the activity of the AMTpolypeptides is reduced or eliminated by transforming a plant cell withan expression cassette comprising a polynucleotide encoding apolypeptide that inhibits the activity of an amt polypeptide. The AMTtransporter activity of an amt polypeptide is inhibited according to thepresent invention if the AMT transporter activity of the AMT polypeptideis less than 70% of the AMT transporter activity of the same AMTpolypeptide in a plant that has not been modified to inhibit the AMTtransporter activity of that AMT polypeptide. In particular embodimentsof the invention, the AMT transporter activity of the AMT polypeptide ina modified plant according to the invention is less than 60%, less than50%, less than 40%, less than 30%, less than 20%, less than 10% or lessthan 5% of the AMT transporter activity of the same AMT polypeptide in aplant that that has not been modified to inhibit the expression of thatAMT polypeptide. The AMT transporter activity of an amt polypeptide is“eliminated” according to the invention when it is not detectable by theassay methods described elsewhere herein. Methods of determining the AMTtransporter activity of an amt polypeptide are described elsewhereherein.

In other embodiments, the activity of an amt polypeptide may be reducedor eliminated by disrupting the gene encoding the AMT polypeptide. Theinvention encompasses mutagenized plants that carry mutations in AMTgenes, where the mutations reduce expression of the AMT gene or inhibitthe AMT transporter activity of the encoded AMT polypeptide.

Thus, many methods may be used to reduce or eliminate the activity of anamt polypeptide. In addition, more than one method may be used to reducethe activity of a single AMT polypeptide. Non-limiting examples ofmethods of reducing or eliminating the expression of AMT polypeptidesare given below.

1. Polynucleotide-Based Methods:

In some embodiments of the present invention, a plant is transformedwith an expression cassette that is capable of expressing apolynucleotide that inhibits the expression of an amt polypeptide of theinvention. The term “expression” as used herein refers to thebiosynthesis of a gene product, including the transcription and/ortranslation of said gene product. For example, for the purposes of thepresent invention, an expression cassette capable of expressing apolynucleotide that inhibits the expression of at least one AMTpolypeptide is an expression cassette capable of producing an RNAmolecule that inhibits the transcription and/or translation of at leastone AMT polypeptide of the invention. The “expression” or “production”of a protein or polypeptide from a DNA molecule refers to thetranscription and translation of the coding sequence to produce theprotein or polypeptide, while the “expression” or “production” of aprotein or polypeptide from an RNA molecule refers to the translation ofthe RNA coding sequence to produce the protein or polypeptide.

Examples of polynucleotides that inhibit the expression of an amtpolypeptide are given below.

i. Sense Suppression/Cosuppression

In some embodiments of the invention, inhibition of the expression of anamt polypeptide may be obtained by sense suppression or cosuppression.For cosuppression, an expression cassette is designed to express an RNAmolecule corresponding to all or part of a messenger RNA encoding an amtpolypeptide in the “sense” orientation. Over expression of the RNAmolecule can result in reduced expression of the native gene.Accordingly, multiple plant lines transformed with the cosuppressionexpression cassette are screened to identify those that show thegreatest inhibition of AMT polypeptide expression.

The polynucleotide used for cosuppression may correspond to all or partof the sequence encoding the AMT polypeptide, all or part of the 5′and/or 3′ untranslated region of an amt polypeptide transcript or all orpart of both the coding sequence and the untranslated regions of atranscript encoding an amt polypeptide. In some embodiments where thepolynucleotide comprises all or part of the coding region for the AMTpolypeptide, the expression cassette is designed to eliminate the startcodon of the polynucleotide so that no protein product will betranslated.

Cosuppression may be used to inhibit the expression of plant genes toproduce plants having undetectable protein levels for the proteinsencoded by these genes. See, for example, Broin, et al., (2002) PlantCell 14:1417-1432. Cosuppression may also be used to inhibit theexpression of multiple proteins in the same plant. See, for example,U.S. Pat. No. 5,942,657. Methods for using cosuppression to inhibit theexpression of endogenous genes in plants are described in Flavell, etal., (1994) Proc. Natl. Acad. Sci. USA 91:3490-3496; Jorgensen, et al.,(1996) Plant Mol. Biol. 31:957-973; Johansen and Carrington, (2001)Plant Physiol. 126:930-938; Broin, et al., (2002) Plant Cell14:1417-1432; Stoutjesdijk, et al., (2002) Plant Physiol. 129:1723-1731;Yu, et al., (2003) Phytochemistry 63:753-763 and U.S. Pat. Nos.5,034,323, 5,283,184 and 5,942,657, each of which is herein incorporatedby reference. The efficiency of cosuppression may be increased byincluding a poly-dT region in the expression cassette at a position 3′to the sense sequence and 5′ of the polyadenylation signal. See, USPatent Application Publication Number 2002/0048814, herein incorporatedby reference. Typically, such a nucleotide sequence has substantialsequence identity to the sequence of the transcript of the endogenousgene, optimally greater than about 65% sequence identity, more optimallygreater than about 85% sequence identity, most optimally greater thanabout 95% sequence identity. See, U.S. Pat. Nos. 5,283,184 and5,034,323, herein incorporated by reference.

ii. Antisense Suppression

In some embodiments of the invention, inhibition of the expression ofthe AMT polypeptide may be obtained by antisense suppression. Forantisense suppression, the expression cassette is designed to express anRNA molecule complementary to all or part of a messenger RNA encodingthe AMT polypeptide. Over expression of the antisense RNA molecule canresult in reduced expression of the native gene. Accordingly, multipleplant lines transformed with the antisense suppression expressioncassette are screened to identify those that show the greatestinhibition of AMT polypeptide expression.

The polynucleotide for use in antisense suppression may correspond toall or part of the complement of the sequence encoding the AMTpolypeptide, all or part of the complement of the 5′ and/or 3′untranslated region of the AMT transcript or all or part of thecomplement of both the coding sequence and the untranslated regions of atranscript encoding the AMT polypeptide. In addition, the antisensepolynucleotide may be fully complementary (i.e., 100% identical to thecomplement of the target sequence) or partially complementary (i.e.,less than 100% identical to the complement of the target sequence) tothe target sequence. Antisense suppression may be used to inhibit theexpression of multiple proteins in the same plant. See, for example,U.S. Pat. No. 5,942,657. Furthermore, portions of the antisensenucleotides may be used to disrupt the expression of the target gene.Generally, sequences of at least 50 nucleotides, 100 nucleotides, 200nucleotides, 300, 400, 450, 500, 550 or greater may be used. Methods forusing antisense suppression to inhibit the expression of endogenousgenes in plants are described, for example, in Liu, et al., (2002) PlantPhysiol. 129:1732-1743 and U.S. Pat. Nos. 5,759,829 and 5,942,657, eachof which is herein incorporated by reference.

Efficiency of antisense suppression may be increased by including apoly-dT region in the expression cassette at a position 3′ to theantisense sequence and 5′ of the polyadenylation signal. See, US PatentApplication Publication Number 2002/0048814, herein incorporated byreference.

iii. Double-Stranded RNA Interference

In some embodiments of the invention, inhibition of the expression of anamt polypeptide may be obtained by double-stranded RNA (dsRNA)interference. For dsRNA interference, a sense RNA molecule like thatdescribed above for cosuppression and an antisense RNA molecule that isfully or partially complementary to the sense RNA molecule are expressedin the same cell, resulting in inhibition of the expression of thecorresponding endogenous messenger RNA.

Expression of the sense and antisense molecules can be accomplished bydesigning the expression cassette to comprise both a sense sequence andan antisense sequence. Alternatively, separate expression cassettes maybe used for the sense and antisense sequences. Multiple plant linestransformed with the dsRNA interference expression cassette orexpression cassettes are then screened to identify plant lines that showthe greatest inhibition of AMT polypeptide expression. Methods for usingdsRNA interference to inhibit the expression of endogenous plant genesare described in Waterhouse, et al., (1998) Proc. Natl. Acad. Sci. USA95:13959-13964, Liu, et al., (2002) Plant Physiol. 129:1732-1743 and WO1999/49029, WO 1999/53050, WO 1999/61631 and WO 2000/49035, each ofwhich is herein incorporated by reference.

iv. Hairpin RNA Interference and Intron-Containing Hairpin RNAInterference

In some embodiments of the invention, inhibition of the expression of anamt polypeptide may be obtained by hairpin RNA (hpRNA) interference orintron-containing hairpin RNA (ihpRNA) interference. These methods arehighly efficient at inhibiting the expression of endogenous genes. See,Waterhouse and Helliwell, (2003) Nat. Rev. Genet. 4:29-38 and thereferences cited therein.

For hpRNA interference, the expression cassette is designed to expressan RNA molecule that hybridizes with itself to form a hairpin structurethat comprises a single-stranded loop region and a base-paired stem. Thebase-paired stem region comprises a sense sequence corresponding to allor part of the endogenous messenger RNA encoding the gene whoseexpression is to be inhibited and an antisense sequence that is fully orpartially complementary to the sense sequence. Alternatively, thebase-paired stem region may correspond to a portion of a promotersequence controlling expression of the gene to be inhibited. Thus, thebase-paired stem region of the molecule generally determines thespecificity of the RNA interference. hpRNA molecules are highlyefficient at inhibiting the expression of endogenous genes and the RNAinterference they induce is inherited by subsequent generations ofplants. See, for example, Chuang and Meyerowitz, (2000) Proc. Natl.Acad. Sci. USA 97:4985-4990; Stoutjesdijk, et al., (2002) Plant Physiol.129:1723-1731 and Waterhouse and Helliwell, (2003) Nat. Rev. Genet.4:29-38. Methods for using hpRNA interference to inhibit or silence theexpression of genes are described, for example, in Chuang andMeyerowitz, (2000) Proc. Natl. Acad. Sci. USA 97:4985-4990;Stoutjesdijk, et al., (2002) Plant Physiol. 129:1723-1731; Waterhouseand Helliwell, (2003) Nat. Rev. Genet. 4:29-38; Pandolfini, et al., BMCBiotechnology 3:7 and US Patent Application Publication Number2003/0175965, each of which is herein incorporated by reference. Atransient assay for the efficiency of hpRNA constructs to silence geneexpression in vivo has been described by Panstruga, et al., (2003) Mol.Biol. Rep. 30:135-140, herein incorporated by reference.

For ihpRNA, the interfering molecules have the same general structure asfor hpRNA, but the RNA molecule additionally comprises an intron that iscapable of being spliced in the cell in which the ihpRNA is expressed.The use of an intron minimizes the size of the loop in the hairpin RNAmolecule following splicing, and this increases the efficiency ofinterference. See, for example, Smith, et al., (2000) Nature407:319-320. In fact, Smith, et al., show 100% suppression of endogenousgene expression using ihpRNA-mediated interference. Methods for usingihpRNA interference to inhibit the expression of endogenous plant genesare described, for example, in Smith, et al., (2000) Nature 407:319-320;Wesley, et al., (2001) Plant J. 27:581-590; Wang and Waterhouse, (2001)Curr. Opin. Plant Biol. 5:146-150; Waterhouse and Helliwell, (2003) Nat.Rev. Genet. 4:29-38; Helliwell and Waterhouse, (2003) Methods30:289-295, and US Patent Application Publication Number 2003/0180945,each of which is herein incorporated by reference.

The expression cassette for hpRNA interference may also be designed suchthat the sense sequence and the antisense sequence do not correspond toan endogenous RNA. In this embodiment, the sense and antisense sequenceflank a loop sequence that comprises a nucleotide sequence correspondingto all or part of the endogenous messenger RNA of the target gene. Thus,it is the loop region that determines the specificity of the RNAinterference. See, for example, WO 2002/00904, Mette, et al., (2000)EMBO J 19:5194-5201; Matzke, et al., (2001) Curr. Opin. Genet. Devel.11:221-227; Scheid, et al., (2002) Proc. Natl. Acad. Sci., USA99:13659-13662; Aufsaftz, et al., (2002) Proc. Nat'l. Acad. Sci.99(4):16499-16506; Sijen, et al., Curr. Biol. (2001) 11:436-440), hereinincorporated by reference.

v. Amplicon-Mediated Interference

Amplicon expression cassettes comprise a plant virus-derived sequencethat contains all or part of the target gene but generally not all ofthe genes of the native virus. The viral sequences present in thetranscription product of the expression cassette allow the transcriptionproduct to direct its own replication. The transcripts produced by theamplicon may be either sense or antisense relative to the targetsequence (i.e., the messenger RNA for the AMT polypeptide). Methods ofusing amplicons to inhibit the expression of endogenous plant genes aredescribed, for example, in Angell and Baulcombe, (1997) EMBO J.16:3675-3684, Angell and Baulcombe, (1999) Plant J. 20:357-362 and U.S.Pat. No. 6,646,805, each of which is herein incorporated by reference.

vi. Ribozymes

In some embodiments, the polynucleotide expressed by the expressioncassette of the invention is catalytic RNA or has ribozyme activityspecific for the messenger RNA of the AMT polypeptide. Thus, thepolynucleotide causes the degradation of the endogenous messenger RNA,resulting in reduced expression of the AMT polypeptide. This method isdescribed, for example, in U.S. Pat. No. 4,987,071, herein incorporatedby reference.

vii. Small Interfering RNA or Micro RNA

In some embodiments of the invention, inhibition of the expression of anamt polypeptide may be obtained by RNA interference by expression of agene encoding a micro RNA (miRNA). miRNAs are regulatory agentsconsisting of about 22 ribonucleotides. miRNA are highly efficient atinhibiting the expression of endogenous genes. See, for example, Javier,et al., (2003) Nature 425:257-263, herein incorporated by reference.

For miRNA interference, the expression cassette is designed to expressan RNA molecule that is modeled on an endogenous miRNA gene. The miRNAgene encodes an RNA that forms a hairpin structure containing a22-nucleotide sequence that is complementary to another endogenous gene(target sequence). For suppression of AMT expression, the 22-nucleotidesequence is selected from an amt transcript sequence and contains 22nucleotides of said AMT sequence in sense orientation and 21 nucleotidesof a corresponding antisense sequence that is complementary to the sensesequence. miRNA molecules are highly efficient at inhibiting theexpression of endogenous genes and the RNA interference they induce isinherited by subsequent generations of plants.

2. Polypeptide-Based Inhibition of Gene Expression

In one embodiment, the polynucleotide encodes a zinc finger protein thatbinds to a gene encoding an amt polypeptide, resulting in reducedexpression of the gene. In particular embodiments, the zinc fingerprotein binds to a regulatory region of an amt gene. In otherembodiments, the zinc finger protein binds to a messenger RNA encodingan amt polypeptide and prevents its translation. Methods of selectingsites for targeting by zinc finger proteins have been described, forexample, in U.S. Pat. No. 6,453,242 and methods for using zinc fingerproteins to inhibit the expression of genes in plants are described, forexample, in US Patent Application Publication Number 2003/0037355, eachof which is herein incorporated by reference.

3. Polypeptide-Based Inhibition of Protein Activity

In some embodiments of the invention, the polynucleotide encodes anantibody that binds to at least one AMT polypeptide and reduces the AMTtransporter activity of the AMT polypeptide. In another embodiment, thebinding of the antibody results in increased turnover of theantibody-AMT complex by cellular quality control mechanisms. Theexpression of antibodies in plant cells and the inhibition of molecularpathways by expression and binding of antibodies to proteins in plantcells are well known in the art. See, for example, Conrad and Sonnewald,(2003) Nature Biotech. 21:35-36, incorporated herein by reference.

4. Gene Disruption

In some embodiments of the present invention, the activity of an amtpolypeptide is reduced or eliminated by disrupting the gene encoding theAMT polypeptide. The gene encoding the AMT polypeptide may be disruptedby any method known in the art. For example, in one embodiment, the geneis disrupted by transposon tagging. In another embodiment, the gene isdisrupted by mutagenizing plants using random or targeted mutagenesis,and selecting for plants that have reduced AMT transporter activity.

i. Transposon Tagging

In one embodiment of the invention, transposon tagging is used to reduceor eliminate the AMT activity of one or more AMT polypeptide. Transposontagging comprises inserting a transposon within an endogenous AMT geneto reduce or eliminate expression of the AMT polypeptide. “AMT gene” isintended to mean the gene that encodes an amt polypeptide according tothe invention.

In this embodiment, the expression of one or more AMT polypeptide isreduced or eliminated by inserting a transposon within a regulatoryregion or coding region of the gene encoding the AMT polypeptide. Atransposon that is within an exon, intron, 5′ or 3′ untranslatedsequence, a promoter or any other regulatory sequence of an amt gene maybe used to reduce or eliminate the expression and/or activity of theencoded AMT polypeptide.

Methods for the transposon tagging of specific genes in plants are wellknown in the art. See, for example, Maes, et al., (1999) Trends PlantSci. 4:90-96; Dharmapuri and Sonti, (1999) FEMS Microbiol. Lett.179:53-59; Meissner, et al., (2000) Plant J. 22:265-274; Phogat, et al.,(2000) J. Biosci. 25:57-63; Walbot, (2000) Curr. Opin. Plant Biol.2:103-107; Gai, et al., (2000) Nucleic Acids Res. 28:94-96; Fitzmaurice,et al., (1999) Genetics 153:1919-1928). In addition, the TUSC processfor selecting Mu insertions in selected genes has been described inBensen, et al., (1995) Plant Cell 7:75-84; Mena, et al., (1996) Science274:1537-1540 and U.S. Pat. No. 5,962,764, each of which is hereinincorporated by reference.

ii. Mutant Plants with Reduced Activity

Additional methods for decreasing or eliminating the expression ofendogenous genes in plants are also known in the art and can besimilarly applied to the instant invention. These methods include otherforms of mutagenesis, such as ethyl methanesulfonate-inducedmutagenesis, deletion mutagenesis and fast neutron deletion mutagenesisused in a reverse genetics sense (with PCR) to identify plant lines inwhich the endogenous gene has been deleted. For examples of thesemethods see, Ohshima, et al., (1998) Virology 243:472-481; Okubara, etal., (1994) Genetics 137:867-874 and Quesada, et al., (2000) Genetics154:421-436, each of which is herein incorporated by reference. Inaddition, a fast and automatable method for screening for chemicallyinduced mutations, TILLING (Targeting Induced Local Lesions In Genomes),using denaturing HPLC or selective endonuclease digestion of selectedPCR products is also applicable to the instant invention. See, McCallum,et al., (2000) Nat. Biotechnol. 18:455-457, herein incorporated byreference.

Mutations that impact gene expression or that interfere with thefunction (AMT transporter activity) of the encoded protein are wellknown in the art. Insertional mutations in gene exons usually result innull-mutants. Mutations in conserved residues are particularly effectivein inhibiting the AMT transporter activity of the encoded protein.Conserved residues of plant AMT polypeptides suitable for mutagenesiswith the goal to eliminate AMT transporter activity have been described.Such mutants can be isolated according to well-known procedures, andmutations in different AMT loci can be stacked by genetic crossing. See,for example, Gruis, et al., (2002) Plant Cell 14:2863-2882.

In another embodiment of this invention, dominant mutants can be used totrigger RNA silencing due to gene inversion and recombination of aduplicated gene locus. See, for example, Kusaba, et al., (2003) PlantCell 15:1455-1467.

The invention encompasses additional methods for reducing or eliminatingthe activity of one or more AMT polypeptide. Examples of other methodsfor altering or mutating a genomic nucleotide sequence in a plant areknown in the art and include, but are not limited to, the use of RNA:DNAvectors, RNA:DNA mutational vectors, RNA:DNA repair vectors,mixed-duplex oligonucleotides, self-complementary RNA:DNAoligonucleotides and recombinogenic oligonucleobases. Such vectors andmethods of use are known in the art. See, for example, U.S. Pat. Nos.5,565,350; 5,731,181; 5,756,325; 5,760,012; 5,795,972 and 5,871,984,each of which are herein incorporated by reference. See also, WO1998/49350, WO 1999/07865, WO 1999/25821 and Beetham, et al., (1999)Proc. Natl. Acad. Sci. USA 96:8774-8778, each of which is hereinincorporated by reference.

iii. Modulating AMT Transporter Activity

In specific methods, the level and/or activity of an amt regulator in aplant is decreased by increasing the level or activity of the AMTpolypeptide in the plant. Methods for increasing the level and/oractivity of AMT polypeptides in a plant are discussed elsewhere herein.Briefly, such methods comprise providing an amt polypeptide of theinvention to a plant and thereby increasing the level and/or activity ofthe AMT polypeptide. In other embodiments, an amt nucleotide sequenceencoding an amt polypeptide can be provided by introducing into theplant a polynucleotide comprising an amt nucleotide sequence of theinvention, expressing the AMT sequence, increasing the activity of theAMT polypeptide and thereby decreasing the ammonium uptake or transportin the plant or plant part. In other embodiments, the AMT nucleotideconstruct introduced into the plant is stably incorporated into thegenome of the plant.

As discussed above, one of skill will recognize the appropriate promoterto use to modulate the level/activity of an amt transporter in theplant. Exemplary promoters for this embodiment have been disclosedelsewhere herein.

Accordingly, the present invention further provides plants having amodified number of cells when compared to the number of cells of acontrol plant tissue. In one embodiment, the plant of the invention hasan increased level/activity of the AMT polypeptide of the invention andthus has an increased Ammonium transport in the plant tissue. In otherembodiments, the plant of the invention has a reduced or eliminatedlevel of the AMT polypeptide of the invention and thus has an increasedNUE in the plant tissue. In other embodiments, such plants have stablyincorporated into their genome a nucleic acid molecule comprising an amtnucleotide sequence of the invention operably linked to a promoter thatdrives expression in the plant cell.

iv. Modulating Root Development

Methods for modulating root development in a plant are provided. By“modulating root development” is intended any alteration in thedevelopment of the plant root when compared to a control plant. Suchalterations in root development include, but are not limited to,alterations in the growth rate of the primary root, the fresh rootweight, the extent of lateral and adventitious root formation, thevasculature system, meristem development or radial expansion.

Methods for modulating root development in a plant are provided. Themethods comprise modulating the level and/or activity of the AMTpolypeptide in the plant. In one method, an amt sequence of theinvention is provided to the plant. In another method, the AMTnucleotide sequence is provided by introducing into the plant apolynucleotide comprising an amt nucleotide sequence of the invention,expressing the AMT sequence and thereby modifying root development. Instill other methods, the AMT nucleotide construct introduced into theplant is stably incorporated into the genome of the plant.

In other methods, root development is modulated by altering the level oractivity of the AMT polypeptide in the plant. A decrease in AMT activitycan result in at least one or more of the following alterations to rootdevelopment, including, but not limited to, larger root meristems,increased in root growth, enhanced radial expansion, an enhancedvasculature system, increased root branching, more adventitious rootsand/or an increase in fresh root weight when compared to a controlplant.

As used herein, “root growth” encompasses all aspects of growth of thedifferent parts that make up the root system at different stages of itsdevelopment in both monocotyledonous and dicotyledonous plants. It is tobe understood that enhanced root growth can result from enhanced growthof one or more of its parts including the primary root, lateral roots,adventitious roots, etc.

Methods of measuring such developmental alterations in the root systemare known in the art. See, for example, US Patent ApplicationPublication Number 2003/0074698 and Werner, et al., (2001) PNAS18:10487-10492, both of which are herein incorporated by reference.

As discussed above, one of skill will recognize the appropriate promoterto use to modulate root development in the plant. Exemplary promotersfor this embodiment include constitutive promoters and root-preferredpromoters. Exemplary root-preferred promoters have been disclosedelsewhere herein.

Stimulating root growth and increasing root mass by decreasing theactivity and/or level of the AMT polypeptide also finds use in improvingthe standability of a plant. The term “resistance to lodging” or“standability” refers to the ability of a plant to fix itself to thesoil. For plants with an erect or semi-erect growth habit, this termalso refers to the ability to maintain an upright position under adverse(environmental) conditions. This trait relates to the size, depth andmorphology of the root system. In addition, stimulating root growth andincreasing root mass by decreasing the level and/or activity of the AMTpolypeptide also finds use in promoting in vitro propagation ofexplants.

Furthermore, higher root biomass production due to a decreased leveland/or activity of AMT activity has a direct effect on the yield and anindirect effect of production of compounds produced by root cells ortransgenic root cells or cell cultures of said transgenic root cells.One example of an interesting compound produced in root cultures isshikonin, the yield of which can be advantageously enhanced by saidmethods.

Accordingly, the present invention further provides plants havingmodulated root development when compared to the root development of acontrol plant. In some embodiments, the plant of the invention has anincreased level/activity of the AMT polypeptide of the invention and hasenhanced root growth and/or root biomass. In other embodiments, suchplants have stably incorporated into their genome a nucleic acidmolecule comprising an amt nucleotide sequence of the invention operablylinked to a promoter that drives expression in the plant cell.

v. Modulating Shoot and Leaf Development

Methods are also provided for modulating shoot and leaf development in aplant. By “modulating shoot and/or leaf development” is intended anyalteration in the development of the plant shoot and/or leaf. Suchalterations in shoot and/or leaf development include, but are notlimited to, alterations in shoot meristem development, in leaf number,leaf size, leaf and stem vasculature, internode length and leafsenescence. As used herein, “leaf development” and “shoot development”encompasses all aspects of growth of the different parts that make upthe leaf system and the shoot system, respectively, at different stagesof their development, both in monocotyledonous and dicotyledonousplants. Methods for measuring such developmental alterations in theshoot and leaf system are known in the art. See, for example, Werner, etal., (2001) PNAS 98:10487-10492 and US Patent Application PublicationNumber 2003/0074698, each of which is herein incorporated by reference.

The method for modulating shoot and/or leaf development in a plantcomprises modulating the activity and/or level of an AMT polypeptide ofthe invention. In one embodiment, an amt sequence of the invention isprovided. In other embodiments, the AMT nucleotide sequence can beprovided by introducing into the plant a polynucleotide comprising anamt nucleotide sequence of the invention, expressing the AMT sequence,and thereby modifying shoot and/or leaf development. In otherembodiments, the AMT nucleotide construct introduced into the plant isstably incorporated into the genome of the plant.

In specific embodiments, shoot or leaf development is modulated byincreasing the level and/or activity of the AMT polypeptide in theplant. An increase in AMT activity can result in at least one or more ofthe following alterations in shoot and/or leaf development, including,but not limited to, reduced leaf number, reduced leaf surface, reducedvascular, shorter internodes and stunted growth and retarded leafsenescence, when compared to a control plant.

As discussed above, one of skill will recognize the appropriate promoterto use to modulate shoot and leaf development of the plant. Exemplarypromoters for this embodiment include constitutive promoters,shoot-preferred promoters, shoot meristem-preferred promoters andleaf-preferred promoters. Exemplary promoters have been disclosedelsewhere herein.

Increasing AMT activity and/or level in a plant results in shorterinternodes and stunted growth. Thus, the methods of the invention finduse in producing dwarf plants. In addition, as discussed above,modulation AMT activity in the plant modulates both root and shootgrowth. Thus, the present invention further provides methods foraltering the root/shoot ratio. Shoot or leaf development can further bemodulated by decreasing the level and/or activity of the AMT polypeptidein the plant.

Accordingly, the present invention further provides plants havingmodulated shoot and/or leaf development when compared to a controlplant. In some embodiments, the plant of the invention has an increasedlevel/activity of the AMT polypeptide of the invention. In otherembodiments, the plant of the invention has a decreased level/activityof the AMT polypeptide of the invention.

vi. Modulating Reproductive Tissue Development

Methods for modulating reproductive tissue development are provided. Inone embodiment, methods are provided to modulate floral development in aplant. By “modulating floral development” is intended any alteration ina structure of a plant's reproductive tissue as compared to a controlplant in which the activity or level of the AMT polypeptide has not beenmodulated. “Modulating floral development” further includes anyalteration in the timing of the development of a plant's reproductivetissue (i.e., a delayed or an accelerated timing of floral development)when compared to a control plant in which the activity or level of theAMT polypeptide has not been modulated. Macroscopic alterations mayinclude changes in size, shape, number or location of reproductiveorgans, the developmental time period that these structures form or theability to maintain or proceed through the flowering process in times ofenvironmental stress. Microscopic alterations may include changes to thetypes or shapes of cells that make up the reproductive organs.

The method for modulating floral development in a plant comprisesmodulating AMT activity in a plant. In one method, an AMT sequence ofthe invention is provided. AN AMT nucleotide sequence can be provided byintroducing into the plant a polynucleotide comprising an amt nucleotidesequence of the invention, expressing the AMT sequence and therebymodifying floral development. In other embodiments, the AMT nucleotideconstruct introduced into the plant is stably incorporated into thegenome of the plant.

In specific methods, floral development is modulated by increasing thelevel or activity of the AMT polypeptide in the plant. An increase inAMT activity can result in at least one or more of the followingalterations in floral development, including, but not limited to,retarded flowering, reduced number of flowers, partial male sterilityand reduced seed set, when compared to a control plant. Inducing delayedflowering or inhibiting flowering can be used to enhance yield in foragecrops such as alfalfa. Methods for measuring such developmentalalterations in floral development are known in the art. See, forexample, Mouradov, et al., (2002) The Plant Cell S111-S130, hereinincorporated by reference.

As discussed above, one of skill will recognize the appropriate promoterto use to modulate floral development of the plant. Exemplary promotersfor this embodiment include constitutive promoters, inducible promoters,shoot-preferred promoters and inflorescence-preferred promoters.

In other methods, floral development is modulated by decreasing thelevel and/or activity of the AMT sequence of the invention. Such methodscan comprise introducing an amt nucleotide sequence into the plant anddecreasing the activity of the AMT polypeptide. In other methods, theAMT nucleotide construct introduced into the plant is stablyincorporated into the genome of the plant. Decreasing expression of theAMT sequence of the invention can modulate floral development duringperiods of stress. Such methods are described elsewhere herein.Accordingly, the present invention further provides plants havingmodulated floral development when compared to the floral development ofa control plant. Compositions include plants having a decreasedlevel/activity of the AMT polypeptide of the invention and having analtered floral development. Compositions also include plants having adecreased level/activity of the AMT polypeptide of the invention whereinthe plant maintains or proceeds through the flowering process in timesof stress.

Methods are also provided for the use of the AMT sequences of theinvention to increase nitrogen use efficiency. The method comprisesdecreasing or increasing the activity of the AMT sequences in a plant orplant part, such as the roots, shoot, epidermal cells, etc.

As discussed above, one of skill will recognize the appropriate promoterto use to manipulate the expression of AMTs. Exemplary promoters of thisembodiment include constitutive promoters, inducible promoters and rootor shoot or leaf preferred promoters.

vii. Method of Use for AMT Promoter Polynucleotides

The polynucleotides comprising the AMT promoters disclosed in thepresent invention, as well as variants and fragments thereof, are usefulin the genetic manipulation of any host cell, preferably plant cell,when assembled with a DNA construct such that the promoter sequence isoperably linked to a nucleotide sequence comprising a polynucleotide ofinterest. In this manner, the AMT promoter polynucleotides of theinvention are provided in expression cassettes along with apolynucleotide sequence of interest for expression in the host cell ofinterest. As discussed in the examples below, the AMT promoter sequencesof the invention are expressed in a variety of tissues and thus thepromoter sequences can find use in regulating the temporal and/or thespatial expression of polynucleotides of interest.

Synthetic hybrid promoter regions are known in the art. Such regionscomprise upstream promoter elements of one polynucleotide operablylinked to the promoter element of another polynucleotide. In anembodiment of the invention, heterologous sequence expression iscontrolled by a synthetic hybrid promoter comprising the AMT promotersequences of the invention, or a variant or fragment thereof, operablylinked to upstream promoter element(s) from a heterologous promoter.Upstream promoter elements that are involved in the plant defense systemhave been identified and may be used to generate a synthetic promoter.See, for example, Rushton, et al., (1998) Curr. Opin. Plant Biol.1:311-315. Alternatively, a synthetic AMT promoter sequence may compriseduplications of the upstream promoter elements found within the AMTpromoter sequences.

It is recognized that the promoter sequence of the invention may be usedwith its native AMT coding sequences. A DNA construct comprising the AMTpromoter operably linked with its native AMT gene may be used totransform any plant of interest to bring about a desired phenotypicchange, such as, modulating root, shoot, leaf, floral and embryodevelopment, stress tolerance and any other phenotype describedelsewhere herein.

The promoter nucleotide sequences and methods disclosed herein areuseful in regulating expression of any heterologous nucleotide sequencein a host plant in order to vary the phenotype of a plant. Variouschanges in phenotype are of interest including modifying the fatty acidcomposition in a plant, altering the amino acid content of a plant,altering a plant's pathogen defense mechanism, and the like. Theseresults can be achieved by providing expression of heterologous productsor increased expression of endogenous products in plants. Alternatively,the results can be achieved by providing for a reduction of expressionof one or more endogenous products, particularly enzymes or cofactors inthe plant. These changes result in a change in phenotype of thetransformed plant.

Genes of interest are reflective of the commercial markets and interestsof those involved in the development of the crop. Crops and markets ofinterest change, and as developing nations open up world markets, newcrops and technologies will emerge also. In addition, as ourunderstanding of agronomic traits and characteristics such as yield andheterosis increase, the choice of genes for transformation will changeaccordingly. General categories of genes of interest include, forexample, those genes involved in information, such as zinc fingers,those involved in communication, such as kinases, and those involved inhousekeeping, such as heat shock proteins. More specific categories oftransgenes, for example, include genes encoding important traits foragronomics, insect resistance, disease resistance, herbicide resistance,sterility, grain characteristics and commercial products. Genes ofinterest include, generally, those involved in oil, starch, carbohydrateor nutrient metabolism as well as those affecting kernel size, sucroseloading, and the like.

In certain embodiments the nucleic acid sequences of the presentinvention can be used in combination (“stacked”) with otherpolynucleotide sequences of interest in order to create plants with adesired phenotype. The combinations generated can include multiplecopies of any one or more of the polynucleotides of interest. Thepolynucleotides of the present invention may be stacked with any gene orcombination of genes to produce plants with a variety of desired traitcombinations, including but not limited to traits desirable for animalfeed such as high oil genes (e.g., U.S. Pat. No. 6,232,529); balancedamino acids (e.g., hordothionins (U.S. Pat. Nos. 5,990,389; 5,885,801;5,885,802 and 5,703,409); barley high lysine (Williamson, et al., (1987)Eur. J. Biochem. 165:99-106 and WO 1998/20122) and high methionineproteins (Pedersen, et al., (1986) J. Biol. Chem. 261:6279; Kirihara, etal., (1988) Gene 71:359 and Musumura, et al., (1989) Plant Mol. Biol.12: 123)); increased digestibility (e.g., modified storage proteins(U.S. patent application Ser. No. 10/053,410, filed Nov. 7, 2001) andthioredoxins (U.S. patent application Ser. No. 10/005,429, filed Dec. 3,2001)), the disclosures of which are herein incorporated by reference.The polynucleotides of the present invention can also be stacked withtraits desirable for insect, disease or herbicide resistance (e.g.,Bacillus thuringiensis toxic proteins (U.S. Pat. Nos. 5,366,892;5,747,450; 5,737,514; 5723,756; 5,593,881; Geiser, et al., (1986) Gene48:109); lectins (Van Damme, et al., (1994) Plant Mol. Biol. 24:825);fumonisin detoxification genes (U.S. Pat. No. 5,792,931); avirulence anddisease resistance genes (Jones, et al., (1994) Science 266:789; Martin,et al., (1993) Science 262:1432; Mindrinos, et al., (1994) Cell78:1089); acetolactate synthase (ALS) mutants that lead to herbicideresistance such as the S4 and/or Hra mutations; inhibitors of glutaminesynthase such as phosphinothricin or basta (e.g., bar gene) andglyphosate resistance (EPSPS gene)) and traits desirable for processingor process products such as high oil (e.g., U.S. Pat. No. 6,232,529);modified oils (e.g., fatty acid desaturase genes (U.S. Pat. No.5,952,544; WO 1994/11516)); modified starches (e.g., ADPGpyrophosphorylases (AGPase), starch synthases (SS), starch branchingenzymes (SBE) and starch debranching enzymes (SDBE)) and polymers orbioplastics (e.g., U.S. Pat. No. 5,602,321; beta-ketothiolase,polyhydroxybutyrate synthase and acetoacetyl-CoA reductase (Schubert, etal., (1988) J. Bacteriol. 170:5837-5847) facilitate expression ofpolyhydroxyalkanoates (PHAs)), the disclosures of which are hereinincorporated by reference. One could also combine the polynucleotides ofthe present invention with polynucleotides affecting agronomic traitssuch as male sterility (e.g., see, U.S. Pat. No. 5,583,210), stalkstrength, flowering time or transformation technology traits such ascell cycle regulation or gene targeting (e.g., WO 1999/61619; WO2000/17364; WO 1999/25821), the disclosures of which are hereinincorporated by reference.

In one embodiment, sequences of interest improve plant growth and/orcrop yields. For example, sequences of interest include agronomicallyimportant genes that result in improved primary or lateral root systems.Such genes include, but are not limited to, nutrient/water transportersand growth induces. Examples of such genes, include but are not limitedto, maize plasma membrane H⁺-ATPase (MHA2) (Frias, et al., (1996) PlantCell 8:1533-44); AKT1, a component of the potassium uptake apparatus inArabidopsis, (Spalding, et al., (1999) J Gen Physiol 113:909-18); RMLgenes which activate cell division cycle in the root apical cells(Cheng, et al., (1995) Plant Physiol 108:881); maize glutaminesynthetase genes (Sukanya, et al., (1994) Plant Mol Biol 26:1935-46) andhemoglobin (Duff, et al., (1997) J. Biol. Chem 27:16749-16752,Arredondo-Peter, et al., (1997) Plant Physiol. 115:1259-1266;Arredondo-Peter, et al., (1997) Plant Physiol 114:493-500 and referencessited therein). The sequence of interest may also be useful inexpressing antisense nucleotide sequences of genes that that negativelyaffects root development.

Additional, agronomically important traits such as oil, starch andprotein content can be genetically altered in addition to usingtraditional breeding methods. Modifications include increasing contentof oleic acid, saturated and unsaturated oils, increasing levels oflysine and sulfur, providing essential amino acids and also modificationof starch. Hordothionin protein modifications are described in U.S. Pat.Nos. 5,703,049, 5,885,801, 5,885,802 and 5,990,389, herein incorporatedby reference. Another example is lysine and/or sulfur rich seed proteinencoded by the soybean 2S albumin described in U.S. Pat. No. 5,850,016and the chymotrypsin inhibitor from barley, described in Williamson, etal., (1987) Eur. J. Biochem. 165:99-106, the disclosures of which areherein incorporated by reference.

Derivatives of the coding sequences can be made by site-directedmutagenesis to increase the level of preselected amino acids in theencoded polypeptide. For example, the gene encoding the barley highlysine polypeptide (BHL) is derived from barley chymotrypsin inhibitor,U.S. patent application Ser. No. 08/740,682, filed Nov. 1, 1996 and WO1998/20133, the disclosures of which are herein incorporated byreference. Other proteins include methionine-rich plant proteins such asfrom sunflower seed (Lilley, et al., (1989) Proceedings of the WorldCongress on Vegetable Protein Utilization in Human Foods and AnimalFeedstuffs, ed. Applewhite (American Oil Chemists Society, Champaign,Ill.), pp. 497-502; herein incorporated by reference); corn (Pedersen,et al., (1986) J. Biol. Chem. 261:6279; Kirihara, et al., (1988) Gene71:359; both of which are herein incorporated by reference) and rice(Musumura, et al., (1989) Plant Mol. Biol. 12:123, herein incorporatedby reference). Other agronomically important genes encode latex, Floury2, growth factors, seed storage factors and transcription factors.

Insect resistance genes may encode resistance to pests that have greatyield drag such as rootworm, cutworm, European Corn Borer and the like.Such genes include, for example, Bacillus thuringiensis toxic proteingenes (U.S. Pat. Nos. 5,366,892; 5,747,450; 5,736,514; 5,723,756;5,593,881 and Geiser, et al., (1986) Gene 48:109); and the like.

Genes encoding disease resistance traits include detoxification genes,such as against fumonosin (U.S. Pat. No. 5,792,931); avirulence (avr)and disease resistance (R) genes (Jones, et al., (1994) Science 266:789;Martin, et al., (1993) Science 262:1432 and Mindrinos, et al., (1994)Cell 78:1089), and the like.

Herbicide resistance traits may include genes coding for resistance toherbicides that act to inhibit the action of acetolactate synthase(ALS), in particular the sulfonylurea-type herbicides (e.g., theacetolactate synthase (ALS) gene containing mutations leading to suchresistance, in particular the S4 and/or Hra mutations), genes coding forresistance to herbicides that act to inhibit action of glutaminesynthase, such as phosphinothricin or basta (e.g., the bar gene) orother such genes known in the art. The bar gene encodes resistance tothe herbicide basta, the nptII gene encodes resistance to theantibiotics kanamycin and geneticin and the ALS-gene mutants encoderesistance to the herbicide chlorsulfuron.

Sterility genes can also be encoded in an expression cassette andprovide an alternative to physical detasseling. Examples of genes usedin such ways include male tissue-preferred genes and genes with malesterility phenotypes such as QM, described in U.S. Pat. No. 5,583,210.Other genes include kinases and those encoding compounds toxic to eithermale or female gametophytic development.

The quality of grain is reflected in traits such as levels and types ofoils, saturated and unsaturated, quality and quantity of essential aminoacids and levels of cellulose. In corn, modified hordothionin proteinsare described in U.S. Pat. Nos. 5,703,049, 5,885,801, 5,885,802 and5,990,389.

Commercial traits can also be encoded on a gene or genes that couldincrease for example, starch for ethanol production or provideexpression of proteins. Another important commercial use of transformedplants is the production of polymers and bioplastics such as describedin U.S. Pat. No. 5,602,321. Genes such as β-Ketothiolase, PHBase(polyhydroxyburyrate synthase) and acetoacetyl-CoA reductase (see,Schubert, et al., (1988) J. Bacteriol. 170:5837-5847) facilitateexpression of polyhyroxyalkanoates (PHAs).

Exogenous products include plant enzymes and products as well as thosefrom other sources including procaryotes and other eukaryotes. Suchproducts include enzymes, cofactors, hormones and the like. The level ofproteins, particularly modified proteins having improved amino aciddistribution to improve the nutrient value of the plant, can beincreased. This is achieved by the expression of such proteins havingenhanced amino acid content.

This invention can be better understood by reference to the followingnon-limiting examples. It will be appreciated by those skilled in theart that other embodiments of the invention may be practiced withoutdeparting from the spirit and the scope of the invention as hereindisclosed and claimed.

EXAMPLES Example 1 Isolation of AMT Sequences

A routine for identifying all members of a given species' ammoniumtransporter (AMT) gene family was employed. First, a diverse set of allthe known available members of the gene family as protein sequences wasprepared from public and proprietary sources. This data could includeorthologous sequences from other species besides these four. Then, as inthe example of maize, these protein query sequences were BLAST algorithmsearched against a combination of proprietary and public maize, genomicor transcript, nucleotide sequence datasets and a non-redundant set ofcandidate AMTs or ‘hits’ was identified. These sequences were combinedwith any existing maize gene family sequences and then curated andedited to arrive at a new working set of unique maize AMT gene ortranscript sequences and their translations. This search for gene familymembers was repeated. If there were recovered new sequences whosenucleotide sequences were unique (not same-gene matches), the processrepeated until completion, that is until no new and distinct nucleotidesequences were found. In this way it was determined that the maize AMTfamily of genes consisted of at least seven members. Eleven distinctsoybean sequences were found. Without the complete genome sequences ofmaize or soybean available, researchers were less certain of the exactgene family size, than they were for Arabidopsis (6 members) and rice(17 members). The availability of complete genome sequences forArabidopsis and rice simplified the search, aided also by availabilityof fairly mature gene models and annotations for these species.

Example 2 Transformation and Regeneration of Transgenic Plants

Immature maize embryos from greenhouse donor plants are bombarded with aplasmid containing the AMT sequence operably linked to thedrought-inducible promoter RAB17 promoter (Vilardell, et al., (1990)Plant Mol Biol 14:423-432) and the selectable marker gene PAT, whichconfers resistance to the herbicide Bialaphos. Alternatively, theselectable marker gene is provided on a separate plasmid. Transformationis performed as follows. Media recipes follow below.

Preparation of Target Tissue

The ears are husked and surface sterilized in 30% Clorox® bleach plus0.5% Micro detergent for 20 minutes, and rinsed two times with sterilewater. The immature embryos are excised and placed embryo axis side down(scutellum side up), 25 embryos per plate, on 560Y medium for 4 hoursand then aligned within the 2.5-cm target zone in preparation forbombardment.

Preparation of DNA

A plasmid vector comprising the AMT sequence operably linked to anubiquitin promoter is made. This plasmid DNA plus plasmid DNA containinga PAT selectable marker is precipitated onto 1.1 μm (average diameter)tungsten pellets using a CaCl₂ precipitation procedure as follows:

100 μl prepared tungsten particles in water

10 μl (1 μg) DNA in Tris EDTA buffer (1 μg total DNA)

100 μl 2.5 M CaCl₂

10 μl 0.1 M spermidine

Each reagent is added sequentially to the tungsten particle suspension,while maintained on the multitube vortexer. The final mixture issonicated briefly and allowed to incubate under constant vortexing for10 minutes. After the precipitation period, the tubes are centrifugedbriefly, liquid removed, washed with 500 ml 100% ethanol and centrifugedfor 30 seconds. Again the liquid is removed, and 105 μl 100% ethanol isadded to the final tungsten particle pellet. For particle gunbombardment, the tungsten/DNA particles are briefly sonicated and 10 μlspotted onto the center of each macrocarrier and allowed to dry about 2minutes before bombardment.

Particle Gun Treatment:

The sample plates are bombarded at level #4 in particle gun #HE34-1 or#HE34-2. All samples receive a single shot at 650 PSI, with a total often aliquots taken from each tube of prepared particles/DNA.

Subsequent Treatment:

Following bombardment, the embryos are kept on 560Y medium for 2 days,then transferred to 560R selection medium containing 3 mg/literBialaphos and subcultured every 2 weeks. After approximately 10 weeks ofselection, selection-resistant callus clones are transferred to 288Jmedium to initiate plant regeneration. Following somatic embryomaturation (2-4 weeks), well-developed somatic embryos are transferredto medium for germination and transferred to the lighted culture room.Approximately 7-10 days later, developing plantlets are transferred to272V hormone-free medium in tubes for 7-10 days until plantlets are wellestablished. Plants are then transferred to inserts in flats (equivalentto 2.5″ pot) containing potting soil and grown for 1 week in a growthchamber, subsequently grown an additional 1-2 weeks in the greenhouse,then transferred to classic 600 pots (1.6 gallon) and grown to maturity.Plants are monitored and scored for increased drought tolerance. Assaysto measure improved drought tolerance are routine in the art andinclude, for example, increased kernel-earring capacity yields underdrought conditions when compared to control maize plants under identicalenvironmental conditions. Alternatively, the transformed plants can bemonitored for a modulation in meristem development (i.e., a decrease inspikelet formation on the ear). See, for example, Bruce, et al., (2002)Journal of Experimental Botany 53:1-13.

Bombardment and Culture Media:

Bombardment medium (560Y) comprises 4.0 g/l N6 basal salts (SIGMAC-1416), 1.0 ml/l Eriksson's Vitamin Mix (1000×SIGMA-1511), 0.5 mg/lthiamine HCl, 120.0 g/l sucrose, 1.0 mg/l 2,4-D and 2.88 g/l L-proline(brought to volume with D-I H₂O following adjustment to pH 5.8 withKOH); 2.0 g/l Gelrite® (added after bringing to volume with D-I H₂O);and 8.5 mg/l silver nitrate (added after sterilizing the medium andcooling to room temperature). Selection medium (560R) comprises 4.0 g/lN6 basal salts (SIGMA C-1416), 1.0 ml/l Eriksson's Vitamin Mix(1000×SIGMA-1511), 0.5 mg/l thiamine HCl, 30.0 g/l sucrose, and 2.0 mg/l2,4-D (brought to volume with D-I H₂O following adjustment to pH 5.8with KOH); 3.0 g/l Gelrite® (added after bringing to volume with D-IH₂O) and 0.85 mg/l silver nitrate and 3.0 mg/l bialaphos (both addedafter sterilizing the medium and cooling to room temperature).

Plant regeneration medium (288J) comprises 4.3 g/l MS salts (GIBCO11117-074), 5.0 ml/l MS vitamins stock solution (0.100 g nicotinic acid,0.02 g/l thiamine HCL, 0.10 g/l pyridoxine HCL, and 0.40 g/l glycinebrought to volume with polished D-I H₂O) (Murashige and Skoog, (1962)Physiol. Plant. 15:473), 100 mg/l myo-inositol, 0.5 mg/l zeatin, 60 g/lsucrose and 1.0 ml/l of 0.1 mM abscisic acid (brought to volume withpolished D-I H₂O after adjusting to pH 5.6); 3.0 g/l Gelrite® (addedafter bringing to volume with D-I H₂O) and 1.0 mg/l indoleacetic acidand 3.0 mg/l bialaphos (added after sterilizing the medium and coolingto 60° C.). Hormone-free medium (272V) comprises 4.3 g/l MS salts (GIBCO11117-074), 5.0 ml/l MS vitamins stock solution (0.100 g/l nicotinicacid, 0.02 g/l thiamine HCL, 0.10 g/l pyridoxine HCL and 0.40 g/lglycine brought to volume with polished D-I H₂O), 0.1 g/l myo-inositoland 40.0 g/l sucrose (brought to volume with polished D-I H₂O afteradjusting pH to 5.6) and 6 g/l Bacto™-agar (added after bringing tovolume with polished D-I H₂O), sterilized and cooled to 60° C.

Example 3 Agrobacterium-Mediated Transformation

For Agrobacterium-mediated transformation of maize with an antisensesequence of the AMT sequence of the present invention, preferably themethod of Zhao is employed (U.S. Pat. No. 5,981,840 and PCT PatentPublication Number WO 1998/32326, the contents of which are herebyincorporated by reference). Briefly, immature embryos are isolated frommaize and the embryos contacted with a suspension of Agrobacterium,where the bacteria are capable of transferring the antisense AMTsequences to at least one cell of at least one of the immature embryos(step 1: the infection step). In this step the immature embryos arepreferably immersed in an Agrobacterium suspension for the initiation ofinoculation. The embryos are co-cultured for a time with theAgrobacterium (step 2: the co-cultivation step). Preferably the immatureembryos are cultured on solid medium following the infection step.Following this co-cultivation period an optional “resting” step iscontemplated. In this resting step, the embryos are incubated in thepresence of at least one antibiotic known to inhibit the growth ofAgrobacterium without the addition of a selective agent for planttransformants (step 3: resting step). Preferably the immature embryosare cultured on solid medium with antibiotic, but without a selectingagent, for elimination of Agrobacterium and for a resting phase for theinfected cells. Next, inoculated embryos are cultured on mediumcontaining a selective agent and growing transformed callus is recovered(step 4: the selection step). Preferably, the immature embryos arecultured on solid medium with a selective agent resulting in theselective growth of transformed cells. The callus is then regeneratedinto plants (step 5: the regeneration step) and preferably calli grownon selective medium are cultured on solid medium to regenerate theplants. Plants are monitored and scored for a modulation in tissuedevelopment.

Example 4 Soybean Embryo Transformation

Soybean embryos are bombarded with a plasmid containing an antisense AMTsequences operably linked to an ubiquitin promoter as follows. To inducesomatic embryos, cotyledons, 3-5 mm in length dissected fromsurface-sterilized, immature seeds of the soybean cultivar A2872, arecultured in the light or dark at 26° C. on an appropriate agar mediumfor six to ten weeks. Somatic embryos producing secondary embryos arethen excised and placed into a suitable liquid medium. After repeatedselection for clusters of somatic embryos that multiplied as early,globular-staged embryos, the suspensions are maintained as describedbelow.

Soybean embryogenic suspension cultures can be maintained in 35 mlliquid media on a rotary shaker, 150 rpm, at 26° C. with florescentlights on a 16:8 hour day/night schedule. Cultures are subcultured everytwo weeks by inoculating approximately 35 mg of tissue into 35 ml ofliquid medium.

Soybean embryogenic suspension cultures may then be transformed by themethod of particle gun bombardment (Klein, et al., (1987) Nature(London) 327:70-73, U.S. Pat. No. 4,945,050). A Du Pont BiolisticPDS1000/HE instrument (helium retrofit) can be used for thesetransformations.

A selectable marker gene that can be used to facilitate soybeantransformation is a transgene composed of the 35S promoter fromCauliflower Mosaic Virus (Odell, et al., (1985) Nature 313:810-812), thehygromycin phosphotransferase gene from plasmid pJR225 (from E. coli;Gritz, et al., (1983) Gene 25:179-188) and the 3′ region of the nopalinesynthase gene from the T-DNA of the Ti plasmid of Agrobacteriumtumefaciens. The expression cassette comprising an antisense AMTsequence operably linked to the ubiquitin promoter can be isolated as arestriction fragment. This fragment can then be inserted into a uniquerestriction site of the vector carrying the marker gene.

To 50 μl of a 60 mg/ml 1 μm gold particle suspension is added (inorder): 5 μl DNA (1 μg/μl), 20 μl spermidine (0.1 M), and 50 μl CaCl₂(2.5 M). The particle preparation is then agitated for three minutes,spun in a microfuge for 10 seconds and the supernatant removed. TheDNA-coated particles are then washed once in 400 μl 70% ethanol andresuspended in 40 μl of anhydrous ethanol. The DNA/particle suspensioncan be sonicated three times for one second each. Five microliters ofthe DNA-coated gold particles are then loaded on each macro carrierdisk.

Approximately 300-400 mg of a two-week-old suspension culture is placedin an empty 60×15 mm petri dish and the residual liquid removed from thetissue with a pipette. For each transformation experiment, approximately5-10 plates of tissue are normally bombarded. Membrane rupture pressureis set at 1100 psi, and the chamber is evacuated to a vacuum of 28inches mercury. The tissue is placed approximately 3.5 inches away fromthe retaining screen and bombarded three times. Following bombardment,the tissue can be divided in half and placed back into liquid andcultured as described above.

Five to seven days post bombardment, the liquid media may be exchangedwith fresh media and eleven to twelve days post-bombardment with freshmedia containing 50 mg/ml hygromycin. This selective media can berefreshed weekly. Seven to eight weeks post-bombardment, green,transformed tissue may be observed growing from untransformed, necroticembryogenic clusters. Isolated green tissue is removed and inoculatedinto individual flasks to generate new, clonally propagated, transformedembryogenic suspension cultures. Each new line may be treated as anindependent transformation event. These suspensions can then besubcultured and maintained as clusters of immature embryos orregenerated into whole plants by maturation and germination ofindividual somatic embryos.

Example 5 Sunflower Meristem Tissue Transformation

Sunflower meristem tissues are transformed with an expression cassettecontaining an antisense AMT sequences operably linked to a ubiquitinpromoter as follows (see also, European Patent Number EP 0 486233,herein incorporated by reference and Malone-Schoneberg, et al., (1994)Plant Science 103:199-207). Mature sunflower seed (Helianthus annuus L.)are dehulled using a single wheat-head thresher. Seeds are surfacesterilized for 30 minutes in a 20% Clorox® bleach solution with theaddition of two drops of Tween® 20 per 50 ml of solution. The seeds arerinsed twice with sterile distilled water.

Split embryonic axis explants are prepared by a modification ofprocedures described by Schrammeijer, et al. (Schrammeijer, et al.,(1990) Plant Cell Rep. 9:55-60). Seeds are imbibed in distilled waterfor 60 minutes following the surface sterilization procedure. Thecotyledons of each seed are then broken off, producing a clean fractureat the plane of the embryonic axis. Following excision of the root tip,the explants are bisected longitudinally between the primordial leaves.The two halves are placed, cut surface up, on GBA medium consisting ofMurashige and Skoog mineral elements (Murashige, et al., (1962) Physiol.Plant., 15:473-497), Shepard's vitamin additions (Shepard, (1980) inEmergent Techniques for the Genetic Improvement of Crops (University ofMinnesota Press, St. Paul, Minn.), 40 mg/l adenine sulfate, 30 g/lsucrose, 0.5 mg/l 6-benzyl-aminopurine (BAP), 0.25 mg/l indole-3-aceticacid (IAA), 0.1 mg/l gibberellic acid (GA₃), pH 5.6, and 8 g/l Phytagar.

The explants are subjected to microprojectile bombardment prior toAgrobacterium treatment (Bidney, et al., (1992) Plant Mol. Biol.18:301-313). Thirty to forty explants are placed in a circle at thecenter of a 60×20 mm plate for this treatment. Approximately 4.7 mg of1.8 mm tungsten microprojectiles are resuspended in 25 ml of sterile TEbuffer (10 mM Tris HCl, 1 mM EDTA, pH 8.0) and 1.5 ml aliquots are usedper bombardment. Each plate is bombarded twice through a 150 mm nytexscreen placed 2 cm above the samples in a PDS 1000® particleacceleration device.

Disarmed Agrobacterium tumefaciens strain EHA105 is used in alltransformation experiments. A binary plasmid vector comprising theexpression cassette that contains the AMT gene operably linked to theubiquitin promoter is introduced into Agrobacterium strain EHA105 viafreeze-thawing as described by Holsters, et al., (1978) Mol. Gen. Genet.163:181-187. This plasmid further comprises a kanamycin selectablemarker gene (i.e, nptII). Bacteria for plant transformation experimentsare grown overnight (28° C. and 100 RPM continuous agitation) in liquidYEP medium (10 gm/l yeast extract, 10 gm/l Bacto®peptone, and 5 gm/lNaCl, pH 7.0) with the appropriate antibiotics required for bacterialstrain and binary plasmid maintenance. The suspension is used when itreaches an OD₆₀₀ of about 0.4 to 0.8. The Agrobacterium cells arepelleted and resuspended at a final OD₆₀₀ of 0.5 in an inoculationmedium comprised of 12.5 mM MES pH 5.7, 1 gm/l NH₄Cl and 0.3 gm/l MgSO₄.

Freshly bombarded explants are placed in an Agrobacterium suspension,mixed, and left undisturbed for 30 minutes. The explants are thentransferred to GBA medium and co-cultivated, cut surface down, at 26° C.and 18-hour days. After three days of co-cultivation, the explants aretransferred to 374B (GBA medium lacking growth regulators and a reducedsucrose level of 1%) supplemented with 250 mg/l cefotaxime and 50 mg/lkanamycin sulfate. The explants are cultured for two to five weeks onselection and then transferred to fresh 374B medium lacking kanamycinfor one to two weeks of continued development. Explants withdifferentiating, antibiotic-resistant areas of growth that have notproduced shoots suitable for excision are transferred to GBA mediumcontaining 250 mg/l cefotaxime for a second 3-day phytohormonetreatment. Leaf samples from green, kanamycin-resistant shoots areassayed for the presence of NPTII by ELISA and for the presence oftransgene expression by assaying for a modulation in meristemdevelopment (i.e., an alteration of size and appearance of shoot andfloral meristems).

NPTII-positive shoots are grafted to Pioneer® hybrid 6440 in vitro-grownsunflower seedling rootstock. Surface sterilized seeds are germinated in48-0 medium (half-strength Murashige and Skoog salts, 0.5% sucrose, 0.3%Gelrite®, pH 5.6) and grown under conditions described for explantculture. The upper portion of the seedling is removed, a 1 cm verticalslice is made in the hypocotyl and the transformed shoot inserted intothe cut. The entire area is wrapped with Parafilm® to secure the shoot.Grafted plants can be transferred to soil following one week of in vitroculture. Grafts in soil are maintained under high humidity conditionsfollowed by a slow acclimatization to the greenhouse environment.Transformed sectors of T₀ plants (parental generation) maturing in thegreenhouse are identified by NPTII ELISA and/or by AMT activity analysisof leaf extracts while transgenic seeds harvested from NPTII-positive T₀plants are identified by AMT activity analysis of small portions of dryseed cotyledon.

An alternative sunflower transformation protocol allows the recovery oftransgenic progeny without the use of chemical selection pressure. Seedsare dehulled and surface-sterilized for 20 minutes in a 20% Clorox®bleach solution with the addition of two to three drops of Tween® 20 per100 ml of solution, then rinsed three times with distilled water.Sterilized seeds are imbibed in the dark at 26° C. for 20 hours onfilter paper moistened with water. The cotyledons and root radical areremoved, and the meristem explants are cultured on 374E (GBA mediumconsisting of MS salts, Shepard vitamins, 40 mg/l adenine sulfate, 3%sucrose, 0.5 mg/l 6-BAP, 0.25 mg/l IAA, 0.1 mg/l GA, and 0.8% Phytagarat pH 5.6) for 24 hours under the dark. The primary leaves are removedto expose the apical meristem, around 40 explants are placed with theapical dome facing upward in a 2 cm circle in the center of 374M (GBAmedium with 1.2% Phytagar), and then cultured on the medium for 24 hoursin the dark.

Approximately 18.8 mg of 1.8 μm tungsten particles are resuspended in150 μl absolute ethanol. After sonication, 8 μl of it is dropped on thecenter of the surface of macrocarrier. Each plate is bombarded twicewith 650 psi rupture discs in the first shelf at 26 mm of Hg helium gunvacuum.

The plasmid of interest is introduced into Agrobacterium tumefaciensstrain EHA105 via freeze thawing as described previously. The pellet ofovernight-grown bacteria at 28° C. in a liquid YEP medium (10 g/l yeastextract, 10 g/l Bacto®peptone, and 5 g/l NaCl, pH 7.0) in the presenceof 50 μg/l kanamycin is resuspended in an inoculation medium (12.5 mM2-mM 2-(N-morpholino) ethanesulfonic acid, MES, 1 g/l NH₄Cl and 0.3 g/lMgSO₄ at pH 5.7) to reach a final concentration of 4.0 at OD₆₀₀.Particle-bombarded explants are transferred to GBA medium (374E) and adroplet of bacteria suspension is placed directly onto the top of themeristem. The explants are co-cultivated on the medium for 4 days, afterwhich the explants are transferred to 374 C medium (GBA with 1% sucroseand no BAP, IAA, GA3 and supplemented with 250 μg/ml cefotaxime). Theplantlets are cultured on the medium for about two weeks under 16-hourday and 26° C. incubation conditions.

Explants (around 2 cm long) from two weeks of culture in 374 C mediumare screened for a modulation in meristem development (i.e., analteration of size and appearance of shoot and floral meristems). Afterpositive (i.e., a decrease in AMT expression) explants are identified,those shoots that fail to exhibit a decrease in AMT activity arediscarded, and every positive explant is subdivided into nodal explants.One nodal explant contains at least one potential node. The nodalsegments are cultured on GBA medium for three to four days to promotethe formation of auxiliary buds from each node. Then they aretransferred to 374 C medium and allowed to develop for an additionalfour weeks. Developing buds are separated and cultured for an additionalfour weeks on 374 C medium. Pooled leaf samples from each newlyrecovered shoot are screened again by the appropriate protein activityassay. At this time, the positive shoots recovered from a single nodewill generally have been enriched in the transgenic sector detected inthe initial assay prior to nodal culture.

Recovered shoots positive for a decreased AMT expression are grafted toPioneer hybrid 6440 in vitro-grown sunflower seedling rootstock. Therootstocks are prepared in the following manner. Seeds are dehulled andsurface-sterilized for 20 minutes in a 20% Clorox® bleach solution withthe addition of two to three drops of Tween® 20 per 100 ml of solution,and are rinsed three times with distilled water. The sterilized seedsare germinated on the filter moistened with water for three days, thenthey are transferred into 48 medium (half-strength MS salt, 0.5%sucrose, 0.3% Gelrite® pH 5.0) and grown at 26° C. under the dark forthree days, then incubated at 16-hour-day culture conditions. The upperportion of selected seedling is removed, a vertical slice is made ineach hypocotyl, and a transformed shoot is inserted into a V-cut. Thecut area is wrapped with Parafilm®. After one week of culture on themedium, grafted plants are transferred to soil. In the first two weeks,they are maintained under high humidity conditions to acclimatize to agreenhouse environment.

Example 6 Identification, Phylogenetic Analysis and ChloroplastTargeting Peptide (cTP) Predictions of AMTs in Arabidopsis, Rice,Soybean and Maize

Taking a ‘genomic’ approach AMTs were identified in several higherplants. In Arabidopsis 6 AMTs have been identified, and phylogeneticanalyses reveals that AtAMT1 (SEQ ID NO: 2) AtAMT1;2 (SEQ ID NO: 4),AtAMT1;3 (SEQ ID NO: 6) and At3g24290 (SEQ ID NO: 10) cluster in onegroup where as AtAMT2 (SEQ ID NO: 8) and At4g28700 (SEQ ID NO: 12) areindependent. Chloroplast targeting peptide (cTP) prediction by ChloroPprogram reveals that AtAMT1;2 (SEQ ID NO: 4) have a putative cTP (with55% probability) where as all other AtAMTs did not contain any predictedcTP In rice, soybean and maize, 17, 11, 7 AMTs have been identified,respectively. cTP prediction in AMTs proteins from maize and soybeandidn't identify any AMT candidate with a putative cTP, however in riceone AMT has putative cTP with more than 50% probability. Phylogeneticanalyses of all the AMTs from Arabidopsis, rice, maize and soybean areshown in FIG. 1.

Example 7 Expression Analysis of AMTs in Maize

In order to identify leaf specific/preferred/expressed AMT(s) in maize,Lynx MPSS expression analyses in ˜300 libraries reveal that ZmAMT1 (SEQID NO: 14), 2, 7 are expressed both in roots and leaves (FIG. 2) whereasZmAMT4 (SEQ ID NO: 20) is a root preferred AMT. ZmAMT6 (SEQ ID NO: 24)expresses at very low level in comparison to other ZmAMTs. In case ofZmAMT5 there was no specific Lynx tag available. Researchers alsoperformed RT-PCR on leaf and roots of B73 maize and the results confirmLynx analysis results that there is no leaf specific AMT in maize,although ZMAMT1,2,7 (SEQ ID NOS: 14, 16 and 26) are expressed in leavesand roots.

Example 8 CTP Predictions in Chloroplast Outer Envelope Proteins

Initial cTP prediction couldn't detect a putative cTP in most of thehigher plant AMTs analyzed. The chloroplast localized AMT (if any) hasto be in the outer envelope of the chloroplast. In order to determinewhether proteins localized in outer envelop of the chloroplast have anypredicted cTP, researchers searched the NCBI database using ‘chloroplastouter envelop/membrane’ as keyword and identified the 14, 14 and 5proteins from Arabidopsis, rice and maize, respectively that are supposeto be localized in outer envelop of chloroplast. Some of these are wellcharacterized proteins and known to be localized in the outer membraneof chloroplast. ChloroP program was used to identify putative cTP inthese 33 candidate proteins and interestingly none of these proteinsshow any putative cTP with high probability. These observations suggestthat either a cTP is not required or not identified/characterized forthese proteins so far. This also suggests that although most of the AMTsdon't have a predicted cTP but some of them might be localized in thechloroplast outer membrane.

Example 9 Isolation and Characterization of AtAMT1;2 (SEQ ID NO: 4)T-DNA Mutant

In cTP prediction analyses, AtAMT1;2 (SEQ ID NO: 4) posses a putativecTP. For functional analyses of AtAMT1;2 (SEQ ID NO: 4) and to determineit's role in N-assimilation, researchers identified a T-DNA mutant line(SM_(—)3.15680) from the Arabidopsis T-DNA mutant data base. The T-DNAmutant line was ordered from ABRC and the homozygous plants weresubjected to molecular analyses. In this mutant line T-DNA was insertedin c-terminal of AtAMT1;2 (SEQ ID NO: 4) gene (FIG. 3A). Genomic PCRsusing AtAMT1;2 (SEQ ID NO: 4) gene and T-DNA specific primers show thatT-DNA is indeed inserted in the AtAMT1;2 (SEQ ID NO: 4) (FIG. 3B).AtAMT1;2 (SEQ ID NO: 4) gene specific primers flanking the T-DNA insertcouldn't amplify any DNA region in mutant plants where as an expectedPCR product was detected in wild type plant (FIG. 4B, upper panel).Similarly, genomic PCR with AtAMT1;2 (SEQ ID NO: 4) specific forwardprimer and T-DNA specific reverse primers amplify an expected product inmutant lines and nothing in wild type plants as expected (FIG. 4B, lowerpanel). Saturated RT-PCRs (35 cycles) analyses couldn't detect a fulllength atamt1;2 mRNA in mutant (FIG. 4C, upper panel) suggesting thatAtAMT1;2 (SEQ ID NO: 4) is completely knocked out in this T-DNA mutant.Actin control RT-PCR worked fine in both mutant and wild type plants(FIG. 3C, lower panel).

Example 10 Generation and Molecular Characterization of AtAMT1;2 (SEQ IDNO: 4) RNAi Lines

In addition to T-DNA mutant, another parallel approach was alsoundertaken for functional analysis of AtAMT1;2 (SEQ ID NO: 4). A RNAivector containing ZM-UBI promoter driven RNAi cassette consisting ofinverted repeats of AtAMT1;2 (SEQ ID NO: 4) specific DNA regions and ADHintron as a spacer was constructed. Wild type Arabidopsis (Columbia-0)was transformed with this RNAi vector by Agrobacterium mediated‘floral-dip’ method. Several transgenic lines were identified byselecting the TO seeds for herbicide resistance in soil. Molecularcharacterization of these transgenic lines were performed by RT-PCR forActin, AtAMT1;2 (SEQ ID NO: 4) RNAi cassette, endogenous AtAMT1;2 (SEQID NO: 4) and presence of gDNA in RNA preparations. Several lines with asignificant reduced levels of AtAMT1;2 (SEQ ID NO: 4) were identifiedafter molecular analysis.

Example 11 Sub-Cellular Localization and Regulation of Expression ofAtAMT1;2 (SEQ ID NO: 4)

cTP prediction analyses indicate that AtAMT1;2 (SEQ ID NO: 4) contains aputative predicted cTP (but with only 55% probability). The objectivesof the experiments described in this example are to determinesub-cellular localization and regulation of expression the endogenousAtAMT1;2 (SEQ ID NO: 4). The coding sequence of AtAMT1;2 (SEQ ID NO: 4)was tagged with green fluorescent protein (GFP) as an in-frameC-terminal fusion under the control of AtAMT1;2 (SEQ ID NO: 4) nativepromoter and a strong constitutive (ZM-UBI) promoter. Arabidopsistransgenic lines were generated and analyzed for GFP expression byconfocal microscopy. Analyses show that AtAMT1;2:GFP is localized in theplasma membrane of endodermis and the cortex in roots.

Example 12 Knock-Out/Knock-Down of Zm-AMTs in Maize

ESTs corresponding to all seven maize AMTs were identified and annotatedand full length cDNA clones were obtained. Experiments toknock-out/knock-down of all these individual ZmAMTs by RNAi are inprogress. TUSC screening experiments were used to identify knock-outmutants for three leaf expressed ZmAMT1 (SEQ ID NO: 14), ZmAMT2 (SEQ IDNO: 16) and ZmAMT7 (SEQ ID NO: 26).

Example 13 Knock-Out/Knock-Down of Multiple AtAMTs with Single RNAiVector in Arabidopsis

Six AMT genes are present in Arabidopsis genome. Hence, it is verylikely that due to functional redundancy one might need to manipulatethe expression of multiple AMTs simultaneously. The DNA sequence of allthese AMTs was analyzed and identified the high homology regions amongthem. For example there is such a stretch of ˜200 bp among AtAMT1;2 (SEQID NO: 4), AtAMT1 (SEQ ID NO: 2), AMT1;3 (SEQ ID NO: 6), At3g24290 (SEQID NO: 10) and At4g28700 (SEQ ID NO: 12) where as AMT2 (SEQ ID NO: 8)stood independent (FIG. 4). These regions were amplified (bold andunderlined in FIG. 4) by PCR from AtAMT1;2 (SEQ ID NO: 4) and AtAMT2(SEQ ID NO: 8) and performed a multi-way ligation to make an invertedrepeat using ADH-intron as a spacer. The RNAi cassette of these hybridinverted repeats is driven by a constitutive or root-specific orleaf-specific promoter. Several transgenic Arabidopsis lines weregenerated for these three constructs. Molecular analyses of these lineswere performed by genomic and RT-PCR. Several lines were identified thatexpressed significantly reduced levels of multiple AtAMTs. Thesetransgenic lines show a methyl ammonium (ammonium analog toxic toplants) tolerant/better growth phenotype as compared to wild typecontrol when grown on MS media supplemented with 10-30 mM of methylammonium. These results indicate multiple AMTs were knocked-down inthese lines, resulting in reduced uptake of methyl ammonium.

Example 14 Knock-Out/Knock-Down of Multiple ZmAMTs in Maize by SingleRNAi Vector

In maize at least 7 AMT like genes were identified and at least 3 ofthem are expressed both in leaf and root (see, Example 2). For improvingNUE by reducing loss of ammonia by volatilization, one might have toknock-out/knock-down multiple AMTs. Detailed analyses of all 7 maizeAMTs were performed to identify the DNA regions showing high homologyamong different ZmAMTs. This analysis reveals that ZmAMT1 (SEQ ID NO:14) and ZmAMT5 (SEQ ID NO: 22), ZmAMT3 (SEQ ID NO: 18) and ZmAMT4 (SEQID NO: 20) and ZmAMT2 (SEQ ID NO: 16), ZmAMT6 (SEQ ID NO: 24) and ZmAMT7(SEQ ID NO: 26) form three separate groups and there is a very highhomology in stretches of DNA sequences with in each group (FIG. 5).Three DNA fragments (bold and underlined in FIG. 5) from ZmAMT 1, 4 and7 (SEQ ID NOS: 14, 20 and 26) representing each of the different groupswere amplified by PCR. Multi-way ligations were performed to makeinverted repeats with hybrid of these 3 fragments and ADH intron as aspacer to facilitate the formation of stem-loop structure. This hybridRNAi cassette of ‘ZmAMT1 (SEQ ID NO: 14):ZmAMT4 (SEQ ID NO: 20):ZmAMT7(SEQ ID NO: 26)’ inverted repeats was driven by Zm-UBI promoter and aleaf-specific promoter. MOPAT driven by Zm-UBI promoter was used asherbicide resistance marker for selected. In addition to that RFP drivenby a pericarp specific promoter LTP2 was also used to sort out thetransgenic seeds (red) from there segregating non-transgenic seeds.Transgenic lines for the constructs were generated, with molecularanalyses of the TO events performed by genomic and RT-PCR. Several lineswith significantly reduced expression of individual/multiple ZmAMTs havebeen identified and characterized.

Example 15 Variants of AMT Sequences

A. Variant Nucleotide Sequences of AMT that do not Alter the EncodedAmino Acid Sequence

The AMT nucleotide sequences are used to generate variant nucleotidesequences having the nucleotide sequence of the open reading frame withabout 70%, 75%, 80%, 85%, 90% and 95% nucleotide sequence identity whencompared to the starting unaltered ORF nucleotide sequence of thecorresponding SEQ ID NO. These functional variants are generated using astandard codon table. While the nucleotide sequence of the variants arealtered, the amino acid sequence encoded by the open reading frames donot change.

B. Variant Amino Acid Sequences of AMT Polypeptides

Variant amino acid sequences of the AMT polypeptides are generated. Inthis example, one amino acid is altered. Specifically, the open readingframes are reviewed to determine the appropriate amino acid alteration.The selection of the amino acid to change is made by consulting theprotein alignment (with the other orthologs and other gene familymembers from various species). An amino acid is selected that is deemednot to be under high selection pressure (not highly conserved) and whichis rather easily substituted by an amino acid with similar chemicalcharacteristics (i.e., similar functional side-chain). Using the proteinalignment set forth in FIG. 2, an appropriate amino acid can be changed.Once the targeted amino acid is identified, the procedure outlined inthe following section C is followed. Variants having about 70%, 75%,80%, 85%, 90% and 95% nucleic acid sequence identity are generated usingthis method.

C. Additional Variant Amino Acid Sequences of AMT Polypeptides

In this example, artificial protein sequences are created having 80%,85%, 90% and 95% identity relative to the reference protein sequence.This latter effort requires identifying conserved and variable regionsfrom the alignment set forth in FIG. 2 and then the judiciousapplication of an amino acid substitutions table. These parts will bediscussed in more detail below.

Largely, the determination of which amino acid sequences are altered ismade based on the conserved regions among AMT protein or among the otherAMT polypeptides. Based on the sequence alignment, the various regionsof the AMT polypeptide that can likely be altered are represented inlower case letters, while the conserved regions are represented bycapital letters. It is recognized that conservative substitutions can bemade in the conserved regions below without altering function. Inaddition, one of skill will understand that functional variants of theAMT sequence of the invention can have minor non-conserved amino acidalterations in the conserved domain.

Artificial protein sequences are then created that are different fromthe original in the intervals of 80-85%, 85-90%, 90-95% and 95-100%identity. Midpoints of these intervals are targeted, with liberallatitude of plus or minus 1%, for example. The amino acids substitutionswill be effected by a custom Perl script. The substitution table isprovided below in Table 2.

TABLE 2 Substitution Table Strongly Similar and Rank of Optimal Order toAmino Acid Substitution Change Comment I L, V 1 50:50 substitution L I,V 2 50:50 substitution V I, L 3 50:50 substitution A G 4 G A 5 D E 6 E D7 W Y 8 Y W 9 S T 10 T S 11 K R 12 R K 13 N Q 14 Q N 15 F Y 16 M L 17First methionine cannot change H Na No good substitutes C Na No goodsubstitutes P Na No good substitutes

First, any conserved amino acids in the protein that should not bechanged is identified and “marked off” for insulation from thesubstitution. The start methionine will of course be added to this listautomatically. Next, the changes are made.

H, C and P are not changed in any circumstance. The changes will occurwith isoleucine first, sweeping N-terminal to C-terminal. Then leucine,and so on down the list until the desired target it reached. Interimnumber substitutions can be made so as not to cause reversal of changes.The list is ordered 1-17, so start with as many isoleucine changes asneeded before leucine, and so on down to methionine. Clearly many aminoacids will in this manner not need to be changed. L, I and V willinvolve a 50:50 substitution of the two alternate optimal substitutions.

The variant amino acid sequences are written as output. Perl script isused to calculate the percent identities. Using this procedure, variantsof the AMT polypeptides are generating having about 80%, 85%, 90%, and95% amino acid identity to the starting unaltered ORF nucleotidesequence of SEQ ID NO: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25,27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61,63, 65, 67, 69, 71, 73, 75, 77, 79 or 81.

Example 16 Over-Expression of AMTs in Plants to Improve NUE

The over-expression of AMTs has been demonstrated with strongconstitutively or organ-specific (e.g. in roots) expression whichimproves ammonium uptake (especially in low ammonium soils in anaerobicconditions typical of rice field conditions) leading to improvednitrogen use efficiency. In other plants, such as maize, typically mostof the N is absorbed by roots in the form of nitrate, the availablesource in most soil, however there is still a considerable proportion ofN available as ammonium. Over-expression of AMTs in these conditionsleads to improved nitrogen utilization. Since nitrate needs to bereduced to ammonium by an energy expensive reaction before it isassimilated, ammonium is a preferable source of N when available to theplant.

All publications and patent applications in this specification areindicative of the level of ordinary skill in the art to which thisinvention pertains. All publications and patent applications are hereinincorporated by reference to the same extent as if each individualpublication or patent application was specifically and individuallyindicated by reference.

The invention has been described with reference to various specific andpreferred embodiments and techniques. However, it should be understoodthat many variations and modifications may be made while remainingwithin the spirit and scope of the invention.

What is claimed is:
 1. An isolated polynucleotide selected from the group consisting of: a. a polynucleotide having at least 70% sequence identity, as determined by the GAP algorithm under default parameters, to the full length sequence of a polynucleotide selected from the group consisting of SEQ ID NOS: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79 or 81; wherein the polynucleotide encodes a polypeptide that functions as a modifier of AMT; b. a polynucleotide encoding a polypeptide selected from the group consisting of SEQ ID NO: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80 or 82; c. a polynucleotide selected from the group consisting of SEQ ID NOS: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79 or 81 and d. a polynucleotide which is complementary to the polynucleotide of (a), (b) or (c).
 2. A recombinant expression cassette, comprising the polynucleotide of claim 1, wherein the polynucleotide is operably linked, in sense or anti-sense orientation, to a promoter.
 3. A host cell comprising the expression cassette of claim
 2. 4. A transgenic plant comprising the recombinant expression cassette of claim
 2. 5. The transgenic plant of claim 4, wherein said plant is a monocot.
 6. The transgenic plant of claim 4, wherein said plant is a dicot.
 7. The transgenic plant of claim 4, wherein said plant is selected from the group consisting of: maize, soybean, sunflower, sorghum, canola, wheat, alfalfa, cotton, rice, barley, millet, peanut, switchgrass, myscanthus, triticale and cocoa.
 8. A transgenic seed from the transgenic plant of claim
 4. 9. A method of modulating the AMT in plants, comprising: a. introducing into a plant cell a recombinant expression cassette comprising the polynucleotide of claim 1 operably linked to a promoter; and b. culturing the plant under plant cell growing conditions; wherein the AMT in said plant cell is modulated.
 10. The method of claim 9, wherein the plant cell is from a plant selected from the group consisting of: maize, soybean, sunflower, sorghum, canola, wheat, alfalfa, cotton, rice, barley, millet, peanut, switchgrass, myscanthus, triticale and cocoa.
 11. A method of modulating the AMT in a plant, comprising: a. introducing into a plant cell a recombinant expression cassette comprising the polynucleotide of claim 1 operably linked to a promoter; b. culturing the plant cell under plant cell growing conditions; and c. regenerating a plant form said plant cell; wherein the AMT in said plant is modulated.
 12. The method of claim 11, wherein the plant is selected from the group consisting of: maize, soybean, sorghum, canola, wheat, alfalfa, cotton, rice, barley, millet, peanut, switchgrass, myscanthus, triticale and cocoa.
 13. A method of decreasing the AMT transporter polypeptide activity in a plant cell, comprising: a. providing a nucleotide sequence comprising at least 15 consecutive nucleotides of the complement of SEQ ID NO: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79 or 81; b. providing a plant cell comprising an mRNA having the sequence set forth in SEQ ID NO: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73, 75, 77, 79 or 81; and c. introducing the nucleotide sequence of step (a) into the plant cell, wherein the nucleotide sequence inhibits expression of the mRNA in the plant cell.
 14. The method of claim 13, wherein said plant cell is from a monocot.
 15. The method of claim 14, wherein said monocot is maize, wheat, rice, barley, sorghum, switchgrass, myscanthus, triticale or rye.
 16. The method of claim 13, wherein said plant cell is from a dicot.
 17. The transgenic plant of claim 4, wherein the AMT transporter activity in said plant is decreased.
 18. The transgenic plant of claim 17, wherein the plant has enhanced root growth.
 19. The transgenic plant of claim 17, wherein the plant has increased seed size.
 20. The transgenic plant of claim 17, wherein the plant has increased seed weight.
 21. The transgenic plant of claim 17, wherein the plant has seed with increased embryo size.
 22. The transgenic plant of claim 17, wherein the plant has increased leaf size.
 23. The transgenic plant of claim 17, wherein the plant has increased seedling vigor.
 24. The transgenic plant of claim 17, wherein the plant has enhanced silk emergence.
 25. The transgenic plant of claim 17, wherein the plant has increased ear size.
 26. The transgenic plant of claim 4, wherein the AMT transporter activity in said plant is increased. 